feat(compiler): implement GDScript lambda expressions from semantics to C codegen

[SuperIceCN]

Summary

Implement GDScript func(...) lambda expressions end-to-end across frontend, LIR, C backend, and runtime helpers. Recorded lambdas in function/constructor bodies now plan captures, synthesize hidden _lambda_<k> shells, lower to construct_lambda, and materialize custom Callables. Property-initializer, parameter-default, match, and await lambda surfaces remain fail-closed.

What changed

• Frontend inventory / capture planning: FrontendVariableAnalyzer.bindLambdaInventory binds lambda parameters, locals, and CAPTURE slots. FrontendLambdaCapturePlanner derives ordered captures (outer PARAMETER / LOCAL / CAPTURE, plus leading self when needed) without capturing class/global/utility names.
• Frontend suite / typing: FrontendSuiteResolver publishes the first complete FrontendLambdaPlan after filling declaration-site capture types and the declared return type. Recorded lambdas type as RESOLVED(GdCallableType); FrontendTypeCheckAnalyzer type-checks lambda bodies as independent callable islands.
• Compile gate: FrontendCompileCheckAnalyzer releases recorded lambdas (published plan + published body) onto the compile surface and recursively scans body facts. Unrecorded lambdas stay fail-closed.
• Lowering: FrontendLoweringFunctionPreparationPass synthesizes hidden static _lambda_<k> shells (is_lambda + is_hidden + is_static). CFG emits LambdaConstructItem; body lowering materializes ConstructLambdaInsn from the frozen plan only.
• LIR: add construct_lambda "<name>" $capture.... LirFunctionDef preserves capture insertion order; DomLirSerializer writes real <capture name type> entries.
• Backend / runtime: ConstructInsnGen.emitConstructLambda copies captures into a heap ${Class}_Capture_${func} block and calls gdcc_new_lambda_callable. CCodegen copies _capture->name into matching locals before __prepare__ and excludes capture locals from default construction. entry.h.ftl generates call / is_valid / free / get_argument_count wrappers.
• Docs: add frontend_lambda_implementation.md as the single fact source and retarget LIR, runtime, compile-gate, lowering, signal, and test-suite docs.
• Tests: focused frontend/backend coverage plus test-suite fixtures under script/lambda/ and validation/lambda/, including signal_connect_lambda.gd.

Why

• Lambda expressions were previously compile-blocked or unsupported, so a core GDScript surface could not be compiled or connected to Signal / Callable consumers.
• Design rationale: freeze one FrontendLambdaPlan per recorded lambda and consume it downstream instead of re-deriving captures. Use a single construct_lambda opcode with self as a leading capture, rather than adding OPCODE_CREATE_SELF_LAMBDA or reusing construct_callable / construct_standalone_callable.
• Capture types come from the outer binding's declaration-site type (including := stabilization), not later assertions. Captures are construction-time copies, matching Godot user-visible semantics while using a userdata ABI.

Affected packages/files

• gd.script.gdcc.frontend.sema.** (FrontendLambdaPlan, FrontendLambdaCapturePlanner, FrontendSuiteResolver, FrontendVariableAnalyzer, FrontendCompileCheckAnalyzer, FrontendTypeCheckAnalyzer)
• gd.script.gdcc.frontend.lowering.** (FrontendLoweringFunctionPreparationPass, LambdaConstructItem, FrontendLambdaConstructInsnLoweringProcessor)
• gd.script.gdcc.lir.** (ConstructLambdaInsn, LirFunctionDef, DomLirSerializer)
• gd.script.gdcc.backend.c.gen.** (ConstructInsnGen, CCodegen, CGenHelper)
• src/main/c/codegen/include_451/gdcc/gdcc_callable.h, src/main/c/codegen/template_451/entry.h.ftl, func.ftl
• doc/module_impl/frontend/frontend_lambda_implementation.md, doc/gdcc_low_ir.md, doc/gdcc_runtime_lib.md
• Test-suite fixtures under src/test/resources/**/script/lambda/ and validation/lambda/

Validation

• pwsh -ExecutionPolicy Bypass -File script/run-gradle-targeted-tests.ps1 -Tests FrontendLambdaCapturePlannerTest,FrontendLambdaSuiteResolutionTest,FrontendLambdaExpressionTypeTest,FrontendLambdaInventoryTest,FrontendLambdaPlanSideTableTest,FrontendLambdaLoweringTest,ConstructLambdaInsnGenTest,FuncHeaderCaptureTemplateTest,FrontendCompileCheckAnalyzerTest,FrontendTypeCheckAnalyzerTest

Result: BUILD SUCCESSFUL (174 tests: 12 capture-planner, 11 suite-resolution, 2 expression-type, 2 inventory, 3 plan side-table, 14 lowering, 10 construct-codegen, 4 capture-template, 67 compile-check, 49 type-check; 0 failures, 0 skipped)

Risks / Notes

• Explicit non-goals: lambdas in property initializers, parameter defaults, or class-level expressions; lambda-owned parameter defaults; match / block-local const / await inside lambda bodies; CAPTURE as a direct-slot alias root; Callable.bind / unbind; ClassDB registration of hidden lambda functions; compiler-only types on lambda params/returns/captures.
• Behavior contract changes: recorded lambdas are now compile-ready. New LIR opcode construct_lambda is added. _lambda_ is a reserved compiler-owned prefix (sema.class_skeleton + skip).
• Deliberate Godot divergence: no GDScriptLambdaSelfCallable; object_id is the cached fat-pointer instance_id only when capturesSelf, otherwise 0. Captures travel in a userdata struct plus a trailing _capture pointer, not as prefixed Godot call arguments. Hash/equal stay Godot's default (call func + userdata identity).
• Missing or conflicting published plans fail-fast in lowering (InvalidInsnException); source errors stay on DiagnosticManager + skip.
• Engine tests (ConstructLambdaInsnGenEngineTest) are environment-aware and skip gracefully when Zig / GODOT_BIN is unavailable.

Key behaviors covered (Optional)

• Nested lambdas transfer outer captures through intermediate layers unless a same-name param/local shadows them.
• self is a leading capture of the enclosing class object type. Signal.connect(func(): self.foo()) uses ObjectDB-checked is_valid_func so the connection invalidates after instance free.
• User-visible arity is the source parameter count; captures are excluded from get_argument_count.
• Capture locals are excluded from __prepare__ default construction, then copied from _capture->name in the prologue.
• Silent local stabilization does not refine var cb := func(): ... slots to Callable.

Diff stats (Optional)

• 107 files changed, 7593 insertions(+), 341 deletions(-)

Breaking changes (Optional)

• None

Related docs (Optional)

• doc/module_impl/frontend/frontend_lambda_implementation.md
• doc/module_impl/frontend/frontend_rules.md
• doc/gdcc_low_ir.md
• doc/gdcc_runtime_lib.md
• doc/module_impl/frontend/frontend_signal_support.md

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