feat(compiler): implement GDScript function parameter default values from semantics to C codegen

[Iridium-Zero]

Summary

Implement source-function parameter default values (func f(a, b = 5):) end-to-end across frontend semantics, lowering, LIR, and C backend binding. Omitted trailing arguments are completed by hidden synthetic default functions; dynamic and engine-side calls use argc-aware callee wrappers rather than bind-time Variant defaults.

What changed

• Frontend sema (FrontendParameterDefaultMetadataOwner, FrontendVisibleValueResolver, FrontendTypeCheckAnalyzer, FrontendCompileCheckAnalyzer): validate default-argument order and variadic constraints; resolve defaults in a dedicated visibility island; publish defaultValueFunc metadata for omitted arguments; type-check accepted defaults; rescan published facts onto the compile surface without duplicating diagnostics on rejected islands.
• Frontend lowering (FrontendLoweringFunctionPreparationPass, CFG/body passes): materialize hidden instance/static shells for accepted defaults, publish PARAMETER_DEFAULT_INIT contexts, and lower expression-rooted CFGs that return the evaluated default. Instance shells take owning-class self; static shells take no parameters.
• Backend C (BindingData, CGenHelper, entry.h.ftl / entry.c.ftl): isolate source defaults from the bind-time Variant channel (default_argument_count stays 0; defaultVariables stays empty and fail-fasts if non-empty). Exact/static calls complete omitted args at the call site; dynamic/engine/virtual dispatch complete them in argc-aware wrappers with per-method userdata (defaultSlotCount, _K_defslot, file-scope <Class>_<method>$default_ud).
• Tests: semantic, compile-gate, lowering, codegen, Zig/Godot integration, and test-suite fixtures under default_args/ (dynamic fill, exact/static, ClassDB.class_call_static, per-call reevaluation, TOO_FEW/TOO_MANY, engine virtual _process(delta: float = 0.0)).
• Docs: replace the rollout plan with frontend_parameter_default_implementation.md as the long-term fact source; retarget frontend, lowering, binding, diagnostic, and test-suite contracts.

Why

• Source functions previously could not omit trailing arguments, so a core GDScript calling convention could not compile.
• Defaults are ordinary expressions that must re-evaluate on each omitted-argument call (mutable array/dictionary defaults must not share a static instance). Bind-time method_info.default_arguments cannot express that, so GDCC keeps ClassDB default-argument count at 0 and completes missing args through synthetic functions.
• Constructor / _init defaults remain permanently out of scope: GDExtension class constructor callbacks have no argument channel and only invoke zero-arg _init.
• Lambda parameter defaults stay fail-closed; they are a separate surface from source-function defaults.

Affected packages/files

• gd.script.gdcc.frontend.sema.analyzer (FrontendParameterDefaultMetadataOwner, FrontendCompileCheckAnalyzer, FrontendTypeCheckAnalyzer, FrontendSuiteResolver, FrontendVariableAnalyzer)
• gd.script.gdcc.frontend.sema.resolver (FrontendVisibleValueResolver)
• gd.script.gdcc.frontend.lowering (FunctionLoweringContext, FrontendLoweringFunctionPreparationPass, CFG/body passes)
• gd.script.gdcc.backend.c.gen (CGenHelper, CCodegen, BindingData)
• src/main/c/codegen/template_451/entry.c.ftl, entry.h.ftl
• Tests — FrontendParameterDefaultMetadataOwnerTest, FrontendCompileCheckAnalyzerTest, FrontendLowering*PassTest, CGenHelperTest, CCodegenTest, CallMethodInsnGenTest, CallStaticMethodInsnGenTest, FrontendLoweringToCProjectBuilderIntegrationTest, GdScriptUnitTestCompileRunnerTest, src/test/test_suite/unit_test/{script,validation}/default_args/
• Docs — doc/module_impl/frontend/frontend_parameter_default_implementation.md, related frontend/backend/diagnostic contracts, doc/gdcc_low_ir.md, doc/test_suite.md

Validation

• script/run-gradle-targeted-tests.sh --tests FrontendParameterDefaultMetadataOwnerTest,FrontendCompileCheckAnalyzerTest,FrontendTypeCheckAnalyzerTest,FrontendLoweringFunctionPreparationPassTest,FrontendLoweringBuildCfgPassTest,FrontendLoweringBodyInsnPassTest,CGenHelperTest,CCodegenTest,CallMethodInsnGenTest,CallStaticMethodInsnGenTest,FrontendLoweringToCProjectBuilderIntegrationTest,GdScriptUnitTestCompileRunnerTest
• ./gradlew classes --no-daemon --info --console=plain
• ./gradlew clean build --no-daemon --console=plain

Result: BUILD SUCCESSFUL

Risks / Notes

• Intentional non-goals: constructor/_init defaults (permanent); lambda parameter defaults; await inside defaults; referencing earlier parameters (func f(a, b = a)); ClassDB method_info.default_arguments registration; signal/@rpc/Callable.bind omitted-argument surfaces; inferring untyped parameter types from defaults (stay Variant); override default inheritance/merging.
• Frozen contracts: LirParameterDef.defaultValueFunc is a hidden synthetic function, not call-site inline-only; defaultVariables is always empty; default_argument_count stays 0; wrapper dedup keys include defaultSlotCount.
• Godot 4.5 GDExtensionMethodBind::call does not pre-check argc; Object.call with too few arguments aborts the current GDScript function. GDCC wrappers complete omitted source defaults before forwarding.
• Direct $ / % in parameter defaults stay fail-closed.

Key behaviors covered

• Required prefix + optional default suffix + optional variadic tail; invalid order and variadic-with-default diagnose sema.invalid_parameter_default_order
• Visibility island: instance defaults may use self/instance members; static defaults may not; parameters/locals/captures/await/get-node diagnose sema.unsupported_parameter_default_expression
• Exact/static calls complete omitted args at the call site; dynamic/ClassDB.class_call_static/engine virtual dispatch complete them in callee wrappers
• Defaults re-evaluate on every omitted-argument call; destroyable OWNED/BORROWED results are cleaned up after the completed call
• TOO_FEW / TOO_MANY arity diagnostics reuse the existing resolver path with published default metadata

Diff stats

• 64 files changed, 4702 insertions(+), 263 deletions(-)

Breaking changes

• None

Related docs

• doc/module_impl/frontend/frontend_parameter_default_implementation.md
• doc/module_impl/frontend/frontend_rules.md
• doc/module_impl/backend/godot_binding_implementation.md
• doc/module_impl/backend/call_method_implementation.md
• doc/module_impl/frontend/diagnostic_manager.md
• doc/gdcc_low_ir.md
• doc/test_suite.md