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.

cmd/comparesrht: daemon entry point, startup validation, dev stub

Wire the compare.sr.ht daemon on core-go's server.New: it runs
crypto.InitCrypto, parses -b/-d/-m/-p, and drives the standard SourceHut
warm-shutdown loop (SIGINT). We deliberately skip WithDefaultMiddleware
(it demands Postgres/Redis and 401s anonymous viewers) and instead
install the web package's documented middleware chain — RealIP, Recoverer,
Logger, config.Middleware, authz.Middleware — inside a Group on the
anonymous router.

validateConfig checks every required key up front (network-key, webhook
key, repos root, a git.sr.ht API origin candidate, meta and compare
origins) with a single clear fatal, so a misconfiguration fails loudly at
startup rather than as a deep panic in config.GetAPI on the first request.

A build-and-exec smoke test proves the binary starts against a synthesized
config, answers /healthz 200, and exits cleanly on SIGINT in under 15s.

Makefile: build ./comparesrht from ./cmd/comparesrht, run-dev binds
localhost:5090 against a local config.ini, install ships the binary and
static assets. contrib/dev-stub is a stdlib-only fake git.sr.ht GraphQL
API returning fixed public repos for local development, and the systemd
unit now stops with KillSignal=SIGINT to hit the warm-shutdown path.

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.