refactor: simplify bg-worker internals + apply review feedback

Combines three semantic units now that the per-scope routing,
catch-all dispatch, and ensure() API have settled.

1. Internals refactor (was two refactor commits):
   - Drops backgroundWorkerRegistry; the per-scope lookup resolves
     directly to the *worker that owns each name. The catch-all
     *worker hosts its lazy-spawned threads in catchAllNames keyed by
     runtimeName.
   - threadbackgroundworker.go gains runtimeName + backgroundReady so
     one catch-all *worker can host multiple per-name threads, each
     carrying its own metric label, FRANKENPHP_WORKER_NAME and
     readiness slot.
   - Worker identity (the runtime name) routes through the CGI
     pipeline so $_SERVER['FRANKENPHP_WORKER'] reflects the
     user-facing name on every request, not just bg-worker boot.

2. Review feedback (dunglas + AlliBalliBaba):
   - BackgroundScope -> Scope (and NextScope / WithRequestScope /
     WithWorkerScope), keeping the type opaque + reusable.
   - backgroundScope -> scope on worker, workerOpt, frankenPHPContext,
     FrankenPHPModule.
   - $_SERVER['FRANKENPHP_WORKER'] now carries the user-facing worker
     name instead of "1"; drops the parallel FRANKENPHP_WORKER_NAME
     injection.
   - Cap-error wording trimmed to read cleanly under JSON config too.
   - ScopeLabel + SetScopeLabel registry; Caddy module resolves a
     human-friendly label per php_server (route host matcher ->
     user-set Caddy server name -> first listener address).
   - reserveBackgroundWorkerThreads extracted from calculateMaxThreads
     so the lazy-start cases (named/named-with-cap/catch-all) read in
     one place.
   - Stub doc nits (third-person godoc); tests adopt
     requireSentinelEventually / require.NoFileExists /
     require.ErrorContains / assert.WithinDuration.
   - background_worker_*_test.go renamed to bgworker*_test.go to
     match the bgworker.go source (no underscores in file-name
     prefixes).

3. Comment cleanup: trims paragraph-length docs in the bg-worker
   subsystem to match the surrounding terseness; ~120 fewer comment
   lines, no semantic changes.

Author: nicolas-grekas