[DeepClone] Drop scoped mode — mangled-only is now the only shape

[nicolas-grekas]

Mirrors symfony/php-ext-deepclone#15 on the polyfill side.

Summary

The scoped shape [$className => ['prop' => $val]] and the flat mangled shape ['prop' => $val, "\0Class\0priv" => $val2] were functionally equivalent; keeping both required a fast path, an intermediate scoped_vars grouping for the MANGLED_VARS branch, a double-pass write, and a footgun guard.

deepclone_hydrate() now interprets $vars exclusively as the flat (array) $obj-shape. Key resolution is done via a cached per-class propertyScopes index (adapted from LazyObjectRegistry::getPropertyScopes() in symfony/var-exporter) that maps every mangled-key shape — bare "foo", "\0*\0foo", "\0Class\0foo" — to [$declaringClass, $realName] in a single hash lookup. Bare names resolve to the most-derived declaring class and correctly target parent-private slots when unambiguous. A scan-based fast path hands $vars straight to the $class hydrator when every key resolves to $class (the common flat-DTO shape), bypassing the intermediate grouping array.

Perf (14-prop DTO, PHP 8.4, warm caches)

shape	before	after	vs Reflection
flat class	3,500 ns	1,930 ns	1.10×
3-level inheritance	4,300 ns	3,760 ns	2.27×

The flat-class fast path is now on par with raw ReflectionProperty::setValue — essentially equal; the inheritance case still pays the per-scope closure dispatch (3 closures × scope-bound), which is orthogonal to this change.

BC breaks

• Scoped-shape input [$class => ['prop' => $val]] is no longer recognized. Callers migrate by flattening: bare names for public / protected / most-derived-private, "\0ParentClass\0prop" for parent-declared private.
• DEEPCLONE_HYDRATE_MANGLED_VARS constant removed (the mode is now implicit). Drop it from $flags arguments.
• DEEPCLONE_HYDRATE_PRESERVE_REFS moves from 1 << 3 to 1 << 2 (filling the slot vacated by MANGLED_VARS). Symbolic references via the constant name are unaffected.
• The "NUL-prefixed scope key → footgun ValueError" guard is gone — NUL-prefix keys are now valid mangled keys.
• A NUL-prefixed key that does not resolve to a declared property on $class or a parent raises ValueError("invalid mangled key") instead of silently creating a dynamic property (matching the ext's "not a parent" rejection).

Tests

• Six scoped-mode-only test cases removed (integer-key / non-array / interface / unrelated / non-existing scope) — the remaining *MatchesUnserialize tests cover the equivalent behavior in the flat shape.
• Added testHydrateBareNameReachesParentPrivate covering bare-name-to-parent-private resolution via the propertyScopes index (GP → P → C chain).
• All 362 DeepClone tests pass locally.

Follow-up (out of scope for this PR)

symfony/var-exporter's Hydrator::hydrate() / Instantiator::instantiate() still need a separate PR to keep their $scopedVars / $mangledVars BC parameters by translating to the flat shape internally.

Test plan

• 362/362 DeepClone tests green locally
• CI green across the PHP matrix