[codex] Move LLVM instruction cost table to v4 cost lens

[briansrls]

Summary

• Move llvm_instruction_cost out of src/v4/extdeps/languages/llvm_ir.dag and into src/v4/lens/cost.dag, so the LLVM instruction cost table is owned by the cost lens rather than the LLVM IR shape model.
• Update the dissolution inventory to mark P9 landed and reduce the pending predicate count accordingly.
• Add an integration ratchet that checks the cost table has a single authority under v4/lens/cost.dag and is not still listed or defined in llvm_ir.dag.

Why

P9 in docs/audit/dissolution-inventory.md identified llvm_instruction_cost as a property-projection table that should be owned by lens/cost.dag / T-12. Keeping it in llvm_ir.dag made LLVM IR both the instruction-shape authority and the cost-table authority.

Validation

• cargo test -p v3-compiler v4_lens_cost_owns_llvm_instruction_cost_table --test integration

Note: a semantic compile smoke for src/v4/lens/cost.dag is not viable yet because the current v4 lowering path rejects block-bodied match functions and single-file v4 sibling imports; the ratchet therefore pins the single-authority placement directly.