feat: declared background workers + frankenphp_get_worker_handle()

[nicolas-grekas]

Summary

Rewrite of #2398 on top of #2499, targeting the refactor/phpserver branch:
the smallest useful slice of the background-worker work, now built on
Server and the server names merged in #2529 instead of a parallel Scope
mechanism. A single commit on top of the base branch.

A worker declared with WithWorkerBackground() / background in the
Caddyfile runs its script in a loop outside the HTTP request cycle. The
frankenphp_get_worker_handle() primitive hands the script a stream that
reaches EOF when FrankenPHP drains the worker, so it can park on
stream_select() and exit gracefully on shutdown, reboot or restart.

What's in

• PHP API: frankenphp_get_worker_handle(): resource, closes when the
  worker is drained (the Go side closes the write end of a per-thread stop
  pipe).
• Go API: WithWorkerBackground(). Background workers attach to a
  Server through the existing WithWorkerServerScope(); num >= 1 is
  required (no lazy-start in this build) and the name is mandatory since it
  is the script's identity ($_SERVER['FRANKENPHP_WORKER']).
• Lifecycle: backgroundWorkerThread implements threadHandler and
  mirrors workerThread's state machine: boot, re-run on cooperative exit
  (status 0, backoff reset), crash-restart with quadratic backoff, hard
  failure on max_consecutive_failures during startup only.
• Caddy: background flag inside worker blocks (php_server and global).
  name is required, match is rejected. Metrics/logs use the worker name,
  which feat: name Server instances and attribute workers to them #2529's qualification already keeps unique across php_server
  blocks, so two blocks can declare the same worker name.

What the rewrite changes vs #2398

Building on Server deletes the parallel machinery the old PR carried and
addresses all review comments:

• Scope, NextScope, SetScopeLabel, the scopeOwners registry and
  caddy/scopelabel.go are gone; Server and the feat: name Server instances and attribute workers to them #2529 name resolution
  replace them. This also removes the reported scopeOwners data race and
  the m#<label>:m#<name> double-prefix in metric names. The feature diff
  is ~250 lines smaller than feat: declared background workers + frankenphp_get_worker_handle() #2398.
• The C side no longer caches a php_stream in a TLS slot: each
  frankenphp_get_worker_handle() call dups the read fd into a fresh
  stream owned by its zval. No GC_ADDREF without release, no dangling
  pointer across restarts or thread recycling, nothing to clean up in
  frankenphp_update_local_thread_context() beyond the raw fds.
• The -1 return of frankenphp_set_background_worker_and_get_stop_fd_write()
  is handled: Init() fails with a clear error during startup; past
  startup the thread backs off and retries instead of letting the script
  crash-loop on frankenphp_get_worker_handle() exceptions.
• The "exponential backoff" comment now says quadratic, matching the
  implementation.
• drain() (the threadHandler seam that is declared but not called yet
  on this branch) is wired into thread.shutdown() and
  rebootAllThreads(), so shutdowns and watch-triggered reboots wake
  parked background workers instead of hitting the force-kill grace period.
• Hardening beyond the review comments: frankenphp_handle_request()
  throws when called from a background worker (previously it would have hit
  the .(*workerThread) type assertion and crashed the process), and
  WithWorkerName() rejects requests targeting a background worker.
• Start/Ready/Stop worker metrics are balanced (background workers count as
  ready once the script starts, and every stop path emits StopWorker), so
  the readyWorkers gauge can't drift negative.

Deferred (kept out for review surface)

• frankenphp_ensure_background_worker() and lazy-start machinery.
• Catch-all (empty-name) workers.
• Shared-state APIs (frankenphp_set_vars / frankenphp_get_vars).
• An orchestrator-style frankenphp_start_background_worker() runtime API
  (discussed in feat: declared background workers + frankenphp_get_worker_handle() #2398); the primitives here are compatible with it.

Test plan

• TestBackgroundWorkerLifecycle: bg worker boots, touches sentinel,
  parks on the stop pipe, Shutdown() returns within 10s.
• TestBackgroundWorkerCrashRestarts: exit(1) on first boot, the
  respawned run touches the "restarted" sentinel.
• TestBackgroundWorkerOnServer: server-scoped worker inherits the
  server env, FRANKENPHP_WORKER carries the name, HTTP requests on the
  same server still serve.
• TestBackgroundWorkerValidation: name required, num >= 1, global
  name namespace, request matchers rejected.
• TestWorkerBackgroundConfig / RequiresName / RejectsMatch
  (Caddyfile parsing, pass locally).

The runtime tests need CI to confirm: on my current box (WSL2), per-thread
engine bootstrap of any embed ZTS build is pathologically slow, so every
Init()-based test times out locally, including on unmodified base commits.

[nicolas-grekas]

Should be considered after #2499 is merged of course!