fix(cac-path-parity): make the FPM proof harness actually runnable as shipped

The artifact cited as the web-SAPI evidence could not have been run as it stood.
Measured in an official php:8.3-fpm container with the extension built in place:

  - as shipped, no args:            "SKIP: php-fpm not found", exit 0.
    The default was `php-fpm8.3`, which matches neither the official images
    (`php-fpm`) nor this repo's images.
  - with the binary supplied by hand: 9 FAIL, every one with an empty `got:`.
    The generated pool had no user/group, so php-fpm refused to start as root
    ("please specify user and group other than root"). A startup failure was
    wearing the costume of nine parity bugs.

Changes:
  - auto-detect the binary (php-fpm, php-fpm8.N, /usr/local/sbin, /usr/sbin) and
    print which one was chosen plus its version;
  - pre-flight the extension with `php-fpm -m`, so a .so that will not load into
    THIS php-fpm reports as a harness failure naming the ABI mismatch rather
    than as nine wrong paths;
  - emit user/group in the pool when running as root, resolved from accounts
    that actually exist (www-data / nobody / daemon), and chmod the fixture tmpdir
    so the non-root worker can read it;
  - run_case() now returns non-zero when php-fpm never answered, and every call
    site routes that to die_startup(), which prints the php-fpm output and the
    pool error_log and exits 2 — an exit code deliberately distinct from 1
    (assertion failure).

After: 9/9 ALL PASS from a clean checkout with no arguments and no environment
fixing, running as root in php:8.3-fpm. Mutation-tested both new paths: a pool
user that does not exist reports "HARNESS FAILURE ... STARTUP/environment
failure" with the real php-fpm error and exit 2; an EXT_SO that is not a loadable
extension is caught by the pre-flight, also exit 2.

Also fixes doc drift: 001-rewrite.phpt pointed at tests/web-sapi-parity-check.sh,
which has never existed. The file it means is tests/fpm-parity-check.sh.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ext/cac-path-parity/tests/001-rewrite.phpt

@@ -15,7 +15,7 @@ HTTP_HOST=site.com
 // PATH_TRANSLATED (to the script path) AFTER the env import, so those three
 // cannot be driven from --ENV-- here. They go through the identical code path
 // as CONTEXT_DOCUMENT_ROOT (one loop over one key table); the real web-SAPI
-// proof for them is tests/web-sapi-parity-check.sh.
+// proof for them is tests/fpm-parity-check.sh.
 var_dump($_SERVER['CONTEXT_DOCUMENT_ROOT']);
 // Non-path vars must be untouched.
 var_dump($_SERVER['HTTP_HOST']);