rename the service to diff.sr.ht

The instance-facing name changes and nothing else does: the config.ini section,
the origin, the apk package, the nginx block and the systemd unit are all
diff.sr.ht now, and the brand's red service label reads "diff" because chrome
derives it from the section name.

What deliberately stays is everything whose rename would cost more than it is
worth: the Go module path, the repository name, the comparesrht binary, and the
/~owner/repo/compare/base...head routes people have in their history. README
says so at the top so the split is not read as an oversight.

web: content-hash the bundle and colour the status column

Two diff-view fixes reported against the deployed instance.

Empty file tree after an upgrade: bundle.js had a stable filename served with a
1-hour cache, so browsers kept a pre-fix bundle and the @pierre/trees sidebar
rendered blank. Content-hash the bundle like the stylesheet — Makefile emits
bundle.<sha256[:8]>.js, server.go globs it into a BundleHref, the templates
reference {{.BundleHref}}, and the hashed name is served immutable. A deploy now
always busts the cache.

Unreadable status column: the changed-files table rendered A/M/D in the dim
default cell colour, near-invisible on the dark chrome. Render each status as a
semantic .diff-status badge (added=green, modified=amber, deleted=red,
renamed=blue) with light/dark variants, via statusClass/statusLabel helpers and
a small self-contained style block (the sourcehut-derived main.min.css can't be
rebuilt locally).

Tests resolve the hashed bundle via a bundleName helper and assert its immutable
cache header; docs updated for the hashed filename.

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.