web: draw the chrome from sr-ht-ecore

The nav/service-switcher, the brand, the login block and the environment
banner were a copy of code every custom service on this instance carries.
The copies drifted, so they now come from one place:
sourcecraft.dev/bigbes/sr-ht-ecore/chrome.

web/chrome.go is gone. New builds one chrome.Service from the shared
config.ini and hands it the discovered stylesheet; view() asks it for a
chrome.Page per request, which viewData embeds rather than copies, so a
field ecore adds later arrives here without an edit. The layout invokes
"srht-env-banner" and "srht-nav" instead of the markup it used to spell
out, the landing's repository listing goes through "srht-repo-list", and
the FuncMap starts from chrome.Funcs() with the local shortsha deleted.

Two ecore policies differ from what this service did and are adopted as
the instance's: the brand label is the config section's short name rather
than a literal "compare", and the origins are trimmed of a trailing
slash. What stays local is what is compare's alone: the bundle href, the
diff-status colours, and the container-fluid the two diff views ask for.

Tests move to testify; the nav ordering/exclusion test goes with the code
it tested, and a test that the layout invokes the chrome at all replaces
it.

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.