Stdlib: gate PHP_ROUND_CEILING/FLOOR/TOWARD_ZERO/AWAY_FROM_ZERO on profile ≥8.4 (#22785)

[PurHur]

Summary

• Gate PHP 8.4-only PHP_ROUND_CEILING / FLOOR / TOWARD_ZERO / AWAY_FROM_ZERO behind CompilerVersion::supportsRoundingModeEnum() via new StdlibConstants::coreIntByName() (also consolidates the existing ARRAY_PAD_* fetch gate).
• defined() / ConstFetch / compile-time fold / JIT helper / get_defined_constants standard bucket honor the gate; internal VmRound still uses the int class constants.
• Closes Regression: PHP_ROUND_CEILING/FLOOR/TOWARD_ZERO/AWAY_FROM_ZERO defined on PROFILE=8.2 — Zend undef (re-#11730/#15083, ext/standard/math.c) #22785.

php-src: ext/standard/php_math_round_mode.h, basic_functions.stub.php (RoundingMode / round modes, 8.4+).

Test plan

• PHP_COMPILER_PROFILE=8.2 php bin/vm.php test/repro/issue_22785_php_round_mode_profile82.php → CEILING/FLOOR/… =0, HALF_UP=1
• PHP_COMPILER_PROFILE=8.4 → all four defined; round(1.1, 0, PHP_ROUND_CEILING) === 2.0
• AOT: bin/compile.php + run under PROFILE=8.2/8.4 for defined('PHP_ROUND_CEILING')
• vendor/bin/phpunit --filter 'php_round_mode_constants|PhpRoundModeConstantsTest' → OK (9 tests)
• php script/bootstrap-inventory.php --check → OK 6508/6508

Verification transcript

# PROFILE=8.2 VM
PHP_ROUND_HALF_UP=1
PHP_ROUND_CEILING=0
PHP_ROUND_FLOOR=0
PHP_ROUND_TOWARD_ZERO=0
PHP_ROUND_AWAY_FROM_ZERO=0

# PROFILE=8.4 VM
PHP_ROUND_CEILING=1 5 … round=2

# AOT
PROFILE=8.4: true / 5
PROFILE=8.2: false / true (HALF_UP)

# phpunit
OK (9 tests, 29 assertions)

Note: AOT fixtures assert defined() + constant values only — native round() AOT currently segfaults/miscompiles independently of this gate (pre-existing).

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[PurHur]

Cleanup bot: deleted leftover remote branch agent/stdlib-22785-php-round-profile-gate (squash-merged here; issue #22785 closed; tip already on master).