Reference const-expr closures through engine ids on PHP 8.6

[nicolas-grekas]

The proposed PHP 8.6 "Serializable closures" engine support (implementation at nicolas-grekas/php-src#4) gives every closure declared in an attribute argument or in a parameter default value a canonical per-class id, derived from a deterministic, non-evaluating walk over the class's constant expressions, exposed as ReflectionFunction::getConstExprId() and resolved by Closure::fromConstExpr(). This PR makes deepclone use those ids, gated on PHP_VERSION_ID >= 80600, assuming the RFC lands.

• Encoding. On PHP 8.6, deepclone_to_array() calls the exported zend_constexpr_closure_ref() and emits [class, id, line] under the existing mask marker. This replaces the per-call declaration-site scan, which had to evaluate every preceding site to count closures: on a 60-site class, encoding the last closure drops from 8.3 us to 1.2 us, and resolving it from 0.3 us scan-equivalents to a 17 ns/site pointer walk. Closures declared in class constant values and in property default values are evaluated in place by the engine and have no id; they keep the site-based 5-element form.
• Decoding. deepclone_from_array() accepts both forms on every PHP version, discriminated by the type of element 1 (int id vs string site). Site-based payloads written on PHP 8.5 keep resolving on PHP 8.6, so caches survive the upgrade; engine-id payloads on older PHP fail with a message saying they need PHP 8.6. Resolution goes through zend_constexpr_closure_site_by_id() plus the same staleness check as before (stale payload when the declaration line moved). Crafted payloads addressing a first-class-callable site are rejected; FCCs keep the named-closure form.
• Gating is unchanged. Closure must be allowed before anything is evaluated on the to_array side, and the payload-named class is allow-list-checked before zend_lookup_class() on the from_array side.
• Runtime closures now surface the engine's own Closure::__serialize() refusal (Serialization of 'Closure' is not allowed) instead of NotInstantiableException, since Closure gains that method on 8.6.

The new code only compiles on PHP >= 8.6, so the current CI matrix is unaffected; the 8.6 paths were validated locally against the patched engine (all phpts pass, and the polyfill suite passes against this build with byte-identical, cross-resolvable payloads). deepclone_constexpr_closures_native.phpt covers the 8.6 behavior, deepclone_constexpr_closures_id_pre86.phpt the refusal on older PHP.

Companion polyfill implementation: symfony/polyfill#633