[Refactor] Extract per-task adstack codegen state and helpers from TaskCodeGenLLVM into TaskCodeGenLLVMAdStack

[duburcqa]

Extract per-task adstack codegen state into TaskCodeGenLLVMAdStack

Stacked on #611. Pure refactor - no behaviour change. Moves all adstack-related per-task fields and helper emission methods out of TaskCodeGenLLVM (quadrants/codegen/llvm/codegen_llvm.h) into a new TaskCodeGenLLVMAdStack class held by composition. The visitors (visit(AdStack*Stmt)) stay on TaskCodeGenLLVM and delegate to adstack_->....

Why

codegen_llvm.h had a ~110-line block of adstack-only state (19 fields: heap strides, per-stack offsets, cached SSA bases, lazy-claim row id, bootstrap pushes, captured bound-expr, ...) plus 9 helper method declarations. codegen_llvm.cpp had the matching 184 field accesses / method definitions interleaved with the rest of the LLVM codegen. Hard to read, hard to extend, dragged the heavy static_adstack_analysis.h / adstack_size_expr.h includes into every translation unit that pulls in codegen_llvm.h.

Mechanism

• New header quadrants/codegen/llvm/codegen_llvm_adstack.h declares TaskCodeGenLLVMAdStack. Owns the 19 state fields and the 9 helper methods (ensure_heap_base_split_llvm, ensure_metadata_llvm, ensure_metadata_split_llvm, ensure_row_id_var_float_llvm, emit_row_claim_llvm, ensure_count_alloca_llvm, emit_single_slot_ptr, get_base_llvm, emit_top_slot_ptr). Holds a TaskCodeGenLLVM& back-reference for accessing builder / entry_block / llvm_context / call / get_runtime / task_codegen_id / llvm_val from the helpers.
• New translation unit quadrants/codegen/llvm/codegen_llvm_adstack.cpp defines the helpers. Bodies are mechanical translations of the originals with builder -> codegen_.builder, entry_block -> codegen_.entry_block, etc.
• codegen_llvm.h: the 110-line state / method block becomes std::unique_ptr<TaskCodeGenLLVMAdStack> adstack_; plus a forward-declared TaskCodeGenLLVMAdStack. ~TaskCodeGenLLVM() switched from = default in the header to an out-of-line definition in the .cpp (PIMPL pattern: required because the unique_ptr holds a forward-declared type, also keeps the heavy adstack includes confined to the .cpp side).
• codegen_llvm.cpp: 184 field-access / method-call references rewired to adstack_->...; the 9 helper definitions deleted (now living in the new .cpp); the constructor adds one line adstack_ = std::make_unique<TaskCodeGenLLVMAdStack>(*this); at the end.
• CMakeLists.txt: new .cpp added to the llvm_codegen target sources.

The is_compile_time_single_slot static helper in codegen_llvm.cpp stays where it is (used by the visit(AdStack*Stmt) visitors that remain on TaskCodeGenLLVM).

Tests

No new tests: pure refactor with mechanical renames. Local Metal serial: 674 passed, 1 skipped, 7 xfailed (same xfail set as origin/main); mpm_grid repro still captures src=reducer_count effective_rows=128 required_bytes=32768. Existing test_adstack_* covers every adstack codegen path through the renamed accessors.

Side-effect audit

• Behaviour-preserving: every field access / method call is renamed, no logic moved between methods, no method bodies edited beyond field -> codegen_.field rewrites.
• ABI-neutral on the LLVM IR side: same LLVMRuntime_get_adstack_* getter calls, same entry_block insertion order, same atomic-rmw / clamp / store sequence at the LCA-block claim site, same per-stack alloca i64 count emission.
• Header-include reduction: every translation unit including codegen_llvm.h no longer transitively pulls static_adstack_analysis.h / adstack_size_expr.h. Compile-time savings are small but real for unrelated TUs (codegen_cpu, codegen_cuda, codegen_amdgpu, kernel_compiler, struct_llvm).

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