feat(compiler): implement PackedArray reference semantics from semantics to C codegen

[Iridium-Zero]

Summary

Align Packed*Array storage and operation semantics with the Godot 4.5 interpreter's reference (shared-identity) semantics across frontend and backend, replacing the old by-value struct (COW) storage model with Variant-backed storage.

What changed

• Backend storage model: every Packed*Array slot (locals, parameters, fields, statics, coroutine frames) is now a godot_Variant; identity flows through godot_new_Variant_with_Variant, and raw struct<->Variant crossing is restricted to the whitelisted helpers in include_451/gdcc/gdcc_packed_ref.h (PackedRefCNames).
• Call emission: builtin methods/operators/constructors keep the native packed ABI and adapt through the internal-pointer getter / wrap_temp (PackedNativeAbiCallSupport); engine method ptrcall helpers materialize packed args into helper-owned slots and wrap packed returns; call_func wrappers preserve identity while ptrcall wrappers intentionally isolate it.
• Iteration: per-family live iterators (intrinsic/for_packed_array_iter.h, GdccForPackedArrayIterType) hold a sharing Variant holder and re-check live size per access — appended elements are visited, shrink/realloc stays safe.
• Frontend writeback: route-provenance-based分流 — DIRECT_SLOT snapshot commits and builtin engine-property mutating-call writebacks are dropped for packed; assignment/subscript routes on the same property still persist; static property and lambda capture routes are unblocked (FrontendWritableTypeWritebackSupport, FrontendCfgGraphBuilder, FrontendLoweringBodyInsnPass).
• Verification: dual-run probe harness (interpreter vs compiled output over the same packed_ref_probes.gd, golden locked by Godot 4.5.2), Godot-independent coverage inventory anchor, fake-engine runtime smoke tests, codegen text anchors, and test_suite end-to-end resources (packed_call_func_identity.gd, packed_ref_full_usage.gd).
• Documentation: consolidated fact source doc/module_impl/backend/packed_array_implementation.md; synchronized gdcc_c_backend.md, gdcc_low_ir.md, gdcc_type_system.md, gdcc_runtime_lib.md, gdcc_ownership_lifecycle_spec.md; code/test comments made self-contained and English-only.

Why

• The old by-value godot_Packed*Array storage copied at the C++ Vector layer and detached on first write, diverging from the interpreter: local aliases, parameter mutations, live for-in iteration, and Variant round-trips were not shared.
• Design trade-offs recorded in the doc: widening writeback on the struct model cannot fix alias sharing, and a gdcc-managed refcount box duplicates the engine's Variant refcounting while still breaking identity at Variant boundaries — both rejected in favor of Variant-backed storage.

Affected packages/files

• src/main/java/gd/script/gdcc/backend/c/** (CGenHelper, PackedRefCNames, PackedNativeAbiCallSupport, CBuiltinBuilder, IndexStoreInsnGen, CBodyBuilder, CHrxIdentityCatalog, ...)
• src/main/java/gd/script/gdcc/frontend/** (writeback route support, CFG builder, lowering pass)
• src/main/c/codegen/include_451/gdcc/gdcc_packed_ref.h, intrinsic/for_packed_array_iter.h, template_451/entry.h.ftl
• src/test/resources/packed_ref_semantics/**, src/test/java/gd/script/gdcc/backend/c/build/packedref/**, src/test/test_suite/unit_test/**
• doc/module_impl/backend/packed_array_implementation.md and the five compiler spec docs

Validation

• script/run-gradle-targeted-tests.sh --tests "PackedArrayReferenceSemanticsDualRunTest,PackedRefSemanticsGoldenInventoryTest,ProbeOutputTest,ProbeGoldenComparisonTest,CConstructInsnGenEngineTest,COperatorInsnGenTest,CConstructInsnGenTest,PackedRefStorageModelSmokeTest,BuiltinCastInsnGenTest,CPackUnpackVariantInsnGenTest,IndexStoreInsnGenTest,IndexStoreInsnGenEngineTest,GdccPackedRefRuntimeSmokeTest,CallMethodInsnGenTest,ExplicitCastSupportTest,CCodegenTest,FrontendLoweringBodyInsnPassTest"
• ./gradlew clean build --no-daemon --info --console=plain

Result: BUILD SUCCESSFUL — 4644 tests, 0 failures (includes Godot 4.5.2 + Zig gated dual-run and integration suites; interpreter and compiled outputs match the full golden line-by-line).

Risks / Notes

• ptrcall ABI boundary intentionally does NOT preserve identity (engine ABI limit; extension-to-extension ptrcall paths only). Ordinary GDScript<->GDExtension calls go through call_func (Variant ABI) and preserve identity; locked by runtime and codegen-anchor tests.
• variant_get_ptr_internal_getter is a semi-internal GDExtension API (also used by godot-cpp); guarded by init-time availability checks with fail-fast.
• Hot reload treats changed packed C-type fingerprints as fail-closed (rejects reload instead of rebinding incorrectly).
• Optional follow-ups recorded in the doc: cleanup of redundant script-property/container-element writebacks, iterator size-eval fusion (measured 12.5 -> 6.9 ns/element potential).

Key behaviors covered (Optional)

• 24-row interpreter-locked semantics matrix: alias/parameter/script-property/static/container-element/signal/lambda/coroutine sharing, live iteration, += rebinding, duplicate() independence, content equality and Dictionary key hashing, same-family as COW copy with fresh identity.
• Builtin engine property: mutating-call not persisted (getter copy), explicit reassignment and subscript write persisted (read-modify-write).
• Supplementary dual-run cases: complex control-flow branch mutation, return-value identity forms, engine method packed arg/return boundary (StreamPeerBuffer.set/get_data_array), PackedStringArray mutation/equality.

Diff stats (Optional)

• 79 files changed, 7451 insertions(+), 366 deletions(-)

Breaking changes (Optional)

• Behavior contract change (intentional, interpreter-aligned): mutating calls on builtin engine properties (e.g. poly.polygon.push_back(x)) no longer persist; the interpreter also discards them. Assignment routes on the same property still persist.
• Internal C ABI change: packed parameter/return/field storage changed from godot_Packed*Array to godot_Variant (internal only; hot-reload fingerprints invalidate old connections fail-closed).

Related docs (Optional)

• doc/module_impl/backend/packed_array_implementation.md
• doc/gdcc_c_backend.md, doc/gdcc_low_ir.md, doc/gdcc_type_system.md, doc/gdcc_runtime_lib.md, doc/gdcc_ownership_lifecycle_spec.md