feat(web): commentable prose diff with honest anchor state (spec-by6.3.3)

Reverses the diff view's founding rule. renderDocDiff skipped ChangeEqual
outright — "the review shows only what changed" — but any block of a proposed
document must be commentable, so unchanged blocks now render as collapsed,
dimmed context. Changed blocks keep their border, tint and full body, so the
page still reads as a diff at a glance rather than as a document dump. A
context block carrying a comment renders open. ChangeMoveIn now shows its text
too, since a comment control on invisible text is a control on nothing;
ChangeMoveOut stays a bare marker and is deliberately not commentable, because
the same paragraph is anchorable at its move-in position and two anchors for
one paragraph is the bug that avoids.

Every rendered block carries id="b-<16 hex>", hashed from the whole anchor
tuple. Not the page ordinal: an ordinal renumbers on any insertion above it, so
a saved link would silently scroll to a neighbouring paragraph, whereas
including the block hash makes a stale link resolve to nothing instead.

Threads are placed by anchor and by nothing else. Anything no rendered block
claims — an outdated anchor, an old-side anchor whose block the diff no longer
draws, a document the proposal no longer changes — is collected into a
page-level "comments that lost their anchor" area. Never dropped, never moved
onto a neighbour: a comment reads as authoritative about the block it sits
beside, so attaching it to the wrong one is worse than admitting it lost its
place. An edited anchor is drawn on its block and badged.

The comment form's anchor is built at submit time from the branch as it now
reads, through service.AnchorOf — hand-rolling the ordinal conversion here
would put the browser's comments on different blocks than the MCP tool's, which
is the one way two surfaces of one conversation disagree without either looking
broken. The form's block hash guards it: a block that moved while the page sat
open is a 409, not a comment attached to whatever took its place.

That hash is required rather than checked-when-present. Skipping the guard for
a form that omits it would let a later template refactor drop the hidden field
and disable the staleness check silently, with every test still green.

Authority is surfaced, not re-implemented: compose and resolve are the owner's
because service says so and ErrForbidden becomes a 403.

spec-by6.3.3

refactor(authn): one Principal.CanRead() for the read-plane ACL (spec-ejq.1)

graph's gate, web's mayRead and mcpsrv's Gate each hand-spelled
'IsOwner() || IsAgent()' — three copies of the read ACL, which graph's own
comment warned is how a corpus leaks when they drift. Define it once as
authn.Principal.CanRead and route all three through it.

coreauth.Derive keeps its own owner||agent test on purpose: it answers a
different question (is this an owner-backed identity to bridge to
AUTH_INTERNAL), and coupling it to the read ACL would misroute a future
read-only viewer kind to the owner's UserID.

Closes spec-ejq.1

feat(web): digest tracks 'since you last looked' via digest_mark (spec-mfm)

The policy-merged digest showed the last N auto-merges by count; the
design intends "what auto-merged since you last saw it", backed by the
digest_mark table that existed but nothing read. The inbox GET now reads
the mark to flag each digest row that merged after it as new, count them,
and draw a divider before the already-seen rows — staying a pure read.

Advancing the mark is a write, so it is an explicit POST /inbox/seen
behind the owner-only + same-origin guard approve/reject already use, not
a side-effecting GET. service.DigestMark/MarkDigestSeen wrap the store,
mapping "no mark yet" to (zero, false) so the first-ever view reads the
whole digest as new.

Closes spec-mfm

feat(web): review queue — inbox + policy-merged digest (Phase 4)

The backstop for work no link reached. /inbox lists every open proposal
on the instance ("waiting on you") and, below it, the digest of recently
policy-merged content — the firehose a human sees after the fact, which
is the whole reason approval=policy is kept distinct from human.

- service.InboxProposals / DigestProposals list instance-wide (one
  reviewer, so a per-space inbox would make them hunt), mapping each
  stored proposal's space_id back to a reference once from the space list.
- web/inbox.go + inbox.html render the two sections; the landing page
  links the queue for a logged-in owner.

Follow-up: the digest currently shows recent policy-merges rather than
"since you last looked" — the digest_mark table exists to track that, but
advancing it is a write and GET stays pure. Filed separately.

feat(web): proposal review page — prose diff + approve/reject (Phase 4)

The browser review plane at /~owner/space/p/<id>, the stable URL every
write already returns. The owner opens the link an agent handed them,
reads a prose diff of each changed document, and approves (merges now) or
rejects.

- web/diff.go: the prose-diff HTML renderer, consuming prosediff's block
  model (the package renders text only; HTML is the web layer's job). It
  implements the Phase 0 verdict's hard requirement — inline word diffs
  above 0.75 block similarity, a two-column old/new view below it, because
  13% of real edits shred and are unreadable inline. All document content
  is HTML-escaped; only the diff structure is markup.
- service/review.go: ProposalDiff reads each changed document's base and
  proposed content for the page to diff (branch tip resolved to a sha, the
  legitimate pinned-rev read, not the ReadDocumentAtRef bypass), and
  MergeHuman fixes the approval kind so a browser approve is always human.
- web/proposal.go: the GET page and the approve/reject POSTs. Only the
  owner may act (an agent is authenticated but has no more approval
  authority than anyone); a cross-site guard on Origin/Referer is the CSRF
  defense a form post needs when the session cookie is meta's. Post-
  redirect-get back to the page. Stale/already-merged approve → 409.
- web.Reader gains the proposal reads and the two actions; the diff-view
  styles go in scss/main.scss (inline marks, two-column, code line diffs).

Inbox and the policy-merged digest are the remaining Phase 4 surfaces.

refactor: make the filter-polarity trap inexpressible

service.SpaceFilter meant "empty membership selects nothing" — a newly
created project has no members — while search.Query.Spaces was a bare
[]core.SpaceRef whose empty case meant every space. Passing a project's
members into a query therefore turned an empty project into the whole
corpus: a silent scope inversion, invisible when it happens, and passing
every test written with a non-empty project.

The filter moves to core.SpaceFilter with unexported fields, and
search.Query takes it whole. There is no slice to hand over any more, so
the inversion cannot be written. Its three states are distinct: every
space (EverythingFilter), exactly these — possibly none — (SpacesFilter),
and the zero value, which is neither. Search refuses the zero value
rather than defaulting it, because both plausible defaults are wrong for
one of the two callers that can produce one, and returns no hits for a
filter that selects no space without asking the index.

service.SpaceFilter is now an alias for the core type, so ResolveProject
and its callers keep their names. Tests that built a Query without a
scope now say core.EverythingFilter(), which is what they always meant.

feat: service.Archive — one accessor, one tree walk, one link graph

web/ and mcpsrv/ both needed a *doc.Archive and each invented its own way
to get one. web/ handed sp.Repo to doc.Scan — reaching past service/ into
gitx, which the layering rule forbids — and then listed the documents a
second time for their bodies, two tree walks per page view. mcpsrv/
converted []service.Document back into []gitx.Document, rebuilding hashes
service/ had already stringified. Two workarounds around one missing
accessor is how three agent-facing surfaces stop being identical.

Service.Archive resolves the revision, walks the tree once, and returns
the archive with the bodies. ArchiveFrom is the same construction over a
Document set a caller already holds — the one place a hex object name is
converted back to a hash, malformed ids refused rather than zeroed.

doc.Page.Links and Page.WordCount were documented as "filled in by a
render pass" and nothing filled them, so Archive.Backlinks always
returned empty; web/ worked around that by re-rendering every document of
the space on each page view. doc.Archive.LinkPass now owns the pass and
the accessor runs it, so the link graph exists wherever the archive does.

web/ and mcpsrv/ no longer import gitx or go-git, in tests either.
doc.DirOf replaces the three copies of the same fromDir helper.

feat: web — the Phase 2 read plane UI and its SCSS entry

SourceHut chrome (nav service-switcher, login block, environment banner,
error page, embedded hashed static assets) ported from compare.sr.ht, plus
the read-plane pages: landing, space document tree, rendered document with
frontmatter/backlinks, and search.

The URL grammar is the design's pinned one: a document's address carries no
extension, ".md" is raw source, ".json" is metadata plus body, and ?rev=
pins any of the three to an immutable revision. Links rendered inside a
pinned page keep the pin, so following one does not silently land on the
approved head.

One human and no visibility levels, so the read ACL is one line: the owner
and its agents read, everyone else is redirected to meta's login (a browser)
or refused with 401 (a client asking for .md/.json).