authz: unified-login identity and git.sr.ht GraphQL authorization

Add the authz package: cookie-derived identity and per-request repository
authorization delegated to git.sr.ht's internal GraphQL API, with no local
database.

- identity.go: UsernameFromRequest decrypts the sr.ht.unified-login.v1 Fernet
  cookie to a bare username ("" for anonymous, never rejects); Middleware/
  ForContext carry it in the request context.
- authz.go: Authorizer interface + GQLAuthorizer over core-go client.Do. Repo
  strips a leading ~ from the owner, maps null user/repository to
  core.ErrNotFound (never leaking existence) and transport/GraphQL failures to
  a wrapped non-NotFound error. MyRepos paginates me{repositories} to a 500-repo
  cap. A mutex-guarded TTL cache memoizes positive and not-found Repo results
  (keyed viewer\0owner\0name) but never transport errors, with lazy expiry and
  opportunistic sweeps.
- Tests: in-memory config (generated Fernet + ed25519 keys) + httptest server
  asserting the Internal auth header decrypts to the expected viewer; covers
  cookie round-trip, found/null/500 repo cases, per-viewer cache keying, TTL
  expiry, transport-error non-caching, and MyRepos pagination.

compare.sr.ht: project foundation — core package, build scaffolding, core-go fork pin

Bootstrap the go.bigb.es/sourcehut-compare service:

- go.mod (go 1.26.4) pinning core-go to the private fork
  git.srht.bigb.es/~bigbes/core-go @ c2c2f3848fa9 via a replace directive;
  go.sum populated by a throwaway smoke build importing core-go
  config/crypto/client/server + chi + logrus (gates API drift from the
  verification baseline fdb3662 to upstream 71b2787).
- core/: pure domain package — sentinel errors, owner/repo/ref validation
  (check-ref-format-style, hostile-input hardened), and the compare-spec
  grammar (ParseCompareSpec, three-dot/two-dot, percent-unescape). Table
  tests at 97% coverage.
- Makefile (all/build/test/css/bundle/run-dev/install), config.example.ini
  documenting the shared keys read in place, contrib/ nginx block and
  systemd unit, README skeleton, .gitignore.

core-go dep deps and go mod tidy are deferred to later phases per plan.