validate_ast: AST uniqueness validator pass

[borisbat]

Summary

• Adds a post-compilation AST validation pass (CodeOfPolicies::validate_ast) that checks every TypeDecl and Expression is unique (not shared) across the AST — catches accidental aliasing bugs early
• Visitor-based duplicate detection + gc_root cross-check for completeness
• Off by default (slow); enable with options validate_ast
• Enables deep-clone optimization in TypeDecl::clone (was commented out) — safe now that uniqueness is enforced
• Fixes several bugs found by the validator: SimNode_AstGetTypeDecl sharing TypeDecls, Variable::source ownership (now weak ref + new loop_source), block folding source sync, apply_qmacro_function sharing returnType, bitfield aliasSubstitution sharing
• Adds Annotation::visitTypeDecls virtual for validator to reach annotation-owned TypeDecls
• Extracts print_backtrace_frames and print_current_stack_trace in crash_handler.h to eliminate duplicated stack trace code from gc_node.cpp (also adds demangling + daScript stack walk callback to gc_break_on_id)
• Updates handmade docs for CodeOfPolicies::validate_ast and Variable::loop_source

Test plan

• 6040 tests pass (dastest -- --test tests/)
• 5455 AOT tests pass (test_aot.exe -use-aot)
• Lint clean (1 minor PERF006 in macro code, not actionable)
• Documentation generated (das2rst.das + Sphinx, zero warnings)
• All .das files formatted

🤖 Generated with Claude Code