feat: persistent-zval helpers (deep-copy zval trees across threads)

[nicolas-grekas]

Second step of the split suggested in #2287: land the persistent-zval
subsystem as a standalone, reviewable header, independent of background
workers. This is the subsystem most likely to hide latent refcount or
memory-lifetime bugs; reviewing it in isolation is higher-signal than
finding issues inside a 3k-line diff.

What

• persistent_zval.h (renamed from the bg_worker_vars.h draft,
  prefix dropped for generality):

  • persistent_zval_validate — whitelist (scalars, arrays of allowed
    values, enum instances). Everything else fails fast.
  • persistent_zval_persist — deep-copy request → persistent (pemalloc)
    memory. Fast paths baked in: interned strings shared, opcache-
    immutable arrays passed by pointer without copying or owning.
  • persistent_zval_free — deep-free; skips interned strings and
    immutable arrays (borrowed, not owned).
  • persistent_zval_to_request — deep-copy persistent → fresh request
    memory. Enums re-resolved by class + case name on each read.

• frankenphp.c: header included only when FRANKENPHP_TEST_HOOKS is
  defined. First real consumer (background workers) drops the guard.

• Test hook gated on FRANKENPHP_TEST_HOOKS:

  • PHP function frankenphp_test_persist_roundtrip(mixed): mixed runs
    validate → persist → to_request → free and returns the result.
  • Registered via zend_register_functions at MINIT so it never
    appears in ext_functions[] and never ships in production builds.

• CI workflows set -DFRANKENPHP_TEST_HOOKS in CGO_CFLAGS
  (tests.yaml + sanitizers.yaml). windows.yaml is the release
  build, not a test runner, and stays untouched.

Notes

• Build verified both without the flag (production path, no
  unused-function warnings) and with it (test path).
• The FRANKENPHP_TEST_HOOKS guard around the header include goes
  away in the PR that lands the first real caller; the test hook
  itself goes away in that same step once end-to-end tests cover the
  code paths.