Stdlib: date_add/date_sub/date_modify/date_diff JIT/AOT lowering (#4604)

[PurHur]

Summary

• Implements JIT/AOT lowering for date_add(), date_sub(), date_modify(), and date_diff() (phase 2 of Stdlib: date_add()/date_sub()/date_modify()/date_diff() — procedural DateTime mutation (ext/date/php_date.c parity) #4604).
• PHP-in-PHP LLVM helpers in lib/JIT/Builtin/DateMutationRuntime.php + ext/standard/JitDateMutation.php mirror VmDateTimeNative / ext/date/php_date.c interval apply, relative modify (compile-time modifier literals), and calendar diff scalars.
• Updates docs/capabilities.md to VM/JIT/AOT yes for all four builtins.

php-src reference

• ext/date/php_date.c — php_date_add, php_date_sub, php_date_diff, date_modify

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter "DateMutationJitCompileTest|date_add"'
# OK (3 tests, 6 assertions)

php bin/compile.php -l test/fixtures/aot/compile-only/date_mutation.php
# exit 0

php bin/compile.php -l test/repro/maintainer_date_add.php
# exit 0

php script/capability-matrix.php
# date_add/date_sub/date_modify/date_diff → yes | yes | yes

Closes #4604

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