Self-host: nested ?? compileExpr + PHPTypes null> fragments (#2276)

[PurHur]

Summary

Lane C picked #2276 after the open implementation-ready language queue in issue % 3 == 2 was entirely blocked (open PRs or stdlib-owned). Bootstrap inventory had regressed to 7 files reporting Expr_BinaryOp_Coalesce plus lib/JIT.php null> PHPDoc fragments.

• lib/Compiler.php: handle Op\Expr\BinaryOp\Coalesce in compileExpr() via compileCoalesce() so nested ?? (e.g. (int) ($stat['mode'] ?? 0) in spine ext/standard/VmFs.php) lowers instead of hitting Unknown BinaryOp Type.
• patches/php-types-generic-null-tail.patch: strip stray null> tails from PHPTypes::fromDecl() when unions inside list<T|null> split incorrectly during docblock reconstruction (lib/JIT.php).

php-src reference

• Zend/zend_compile.c — zend_compile_coalesce() / ZEND_COALESCE (expression lowering)
• Zend/zend_operators.c — ?? runtime (VM already had block-level path; this wires nested expr path)

Verification

./script/docker-exec.sh -- bash -lc './phpc lint --bootstrap-inventory'
# bootstrap-inventory: 0 files with unsupported syntax (854 scanned)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter "CoalesceVmTest::testNullCoalesceNestedInCast|PhpcLintBootstrapInventoryTest"'

Note: CoalesceVmTest::testNullCoalesceThrow still errors on master (pre-existing); not introduced here.

Closes #2276

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