feat: shared-state APIs (set_vars/get_vars) + readiness wait

Adds the worker-to-HTTP shared-state surface deferred from the
config+ensure split (php#2393):

- frankenphp_set_vars(array $vars): void publishes a snapshot from a
  background worker. Persistent (pemalloc) memory, RWMutex-protected,
  cross-thread safe. Skips work when data is identical (=== check).
- frankenphp_get_vars(string $name): array reads the latest snapshot.
  Pure read; throws if the worker is not running or has not published
  yet.
- ensure_background_worker now blocks until the named worker has called
  set_vars at least once (the readiness signal). The fire-and-forget
  semantics from the config-only PR become a stronger contract here
  with no API change visible to callers.
- Two-mode ensure: fail-fast in HTTP-worker bootstrap (before
  frankenphp_handle_request) so a broken dependency surfaces at boot
  rather than serving degraded traffic; tolerant inside requests so
  the restart-with-backoff cycle can recover from transient boot
  failures.
- Boot-failure capture: the worker's last PHP error (message, file,
  line, exit status) is recorded so ensure() can throw a descriptive
  RuntimeException on timeout.

The persistent storage path uses opcache-immutable arrays (zero-copy
share), interned strings (no copy), and rich type support: null,
scalars, arrays (nested), enums.

Tests cover happy-path roundtrips, type coverage, ensure() blocking
on first set_vars, fail-fast vs tolerant modes, boot-failure
reporting, and the catch-all + scope interactions with vars.

Author: nicolas-grekas

.github/workflows/sanitizers.yaml

@@ -108,7 +108,7 @@ jobs:
       - name: Set CGO flags
         run: |
           {
-            echo "CGO_CFLAGS=$CFLAGS -I${PWD}/watcher/target/include -DFRANKENPHP_TEST $(php-config --includes)"
+            echo "CGO_CFLAGS=$CFLAGS -I${PWD}/watcher/target/include $(php-config --includes)"
             echo "CGO_LDFLAGS=$LDFLAGS $(php-config --ldflags) $(php-config --libs)"
           } >> "$GITHUB_ENV"
       - name: Run tests

.github/workflows/tests.yaml

@@ -62,7 +62,7 @@ jobs:
         # TODO: remove this workaround when fixed upstream
         run: sudo apt-get install --reinstall -y libbrotli-dev
       - name: Set CGO flags
-        run: echo "CGO_CFLAGS=-I${PWD}/watcher/target/include -DFRANKENPHP_TEST $(php-config --includes)" >> "${GITHUB_ENV}"
+        run: echo "CGO_CFLAGS=-I${PWD}/watcher/target/include $(php-config --includes)" >> "${GITHUB_ENV}"
       - name: Build
         run: go build
       - name: Build testcli binary
@@ -138,7 +138,7 @@ jobs:
           echo "GEN_STUB_SCRIPT=${PWD}/php-${PHP_VERSION}/build/gen_stub.php" >> "${GITHUB_ENV}"
       - name: Set CGO flags
         run: |
-          echo "CGO_CFLAGS=-DFRANKENPHP_TEST $(php-config --includes)" >> "${GITHUB_ENV}"
+          echo "CGO_CFLAGS=$(php-config --includes)" >> "${GITHUB_ENV}"
           echo "CGO_LDFLAGS=$(php-config --ldflags) $(php-config --libs)" >> "${GITHUB_ENV}"
       - name: Install gotestsum
         run: go install gotest.tools/gotestsum@latest
@@ -175,7 +175,7 @@ jobs:
       - name: Set CGO flags
         run: |
           {
-           echo "CGO_CFLAGS=-I/opt/homebrew/include/ -DFRANKENPHP_TEST $(php-config --includes)"
+           echo "CGO_CFLAGS=-I/opt/homebrew/include/ $(php-config --includes)"
            echo "CGO_LDFLAGS=-L/opt/homebrew/lib/ $(php-config --ldflags) $(php-config --libs)"
           } >> "${GITHUB_ENV}"
       - name: Build