consolidate(codegen): update docs and boundary rules for schema extraction

Update module header in mod.rs from five to six files with schema.rs
description and explicit exclusion of cell operation policy. Replace
legacy monolith description in CODING_STYLE.md with current multi-file
backend structure. Add module boundary rule: schema.rs owns layout
computation and field access, must not absorb cell operation policy
or state-transition verification.

Author: a19q3

CODING_STYLE.md

@@ -23,10 +23,13 @@ project contract.
 
 ## Backend And Codegen Rules
 
-`src/codegen/mod.rs` is currently a legacy monolith covering layout planning,
-CKB syscall lowering, verifier pattern emission, RISC-V assembly generation, the
-internal ELF assembler, and backend-shape tests. New code should reduce coupling
-where practical and must not make the implicit backend contracts more implicit.
+`src/codegen/mod.rs` is the orchestration layer of a multi-file backend.
+Sub-modules handle separate concerns: `schema.rs` (layout data model and
+type-width helpers), `assembler.rs` (RISC-V machine code and ELF),
+`runtime.rs` (helper functions and CKB syscall wrappers), `abi.rs` (calling
+convention and entry witness envelope), and `collections.rs` (collection
+lowering). New code should respect these boundaries and must not make the
+implicit backend contracts more implicit.
 
 - Treat emitted assembly as a compiler contract. Any new mnemonic or pseudo-op
   emitted by codegen, stdlib, or collection helpers must be supported by the
@@ -142,3 +145,22 @@ sub-module (e.g. `assembler.rs`, `runtime.rs`, `abi.rs`):
 5. **Brace-count after every deletion.** Use `python3 -c` to verify brace
    balance before attempting compilation. Off-by-one `sed` ranges can leave
    orphaned lines or eat closing braces.
+
+### Module Boundary: Schema vs Cell Operations
+
+`schema.rs` owns layout computation and field access helpers. It must **not**
+absorb cell operation policy or state-transition verification. Specifically:
+
+- **Schema module may contain**: type-width helpers (`fixed_scalar_width`,
+  `fixed_byte_width`, `type_static_length`, etc.), aggregate/tuple layout
+  computation, Molecule table field bounds/span helpers, fixed-byte comparison
+  and loading, prelude u64 value resolution, and field access dispatch.
+- **Schema module must not contain**: destruction policy, identity/field
+  uniqueness checks, create-output field verification, state-transition edge
+  matching, consume/destroy/replace/transfer/settle lowering, mutate
+  replacement transition checks, or any code that decides *whether* a cell
+  operation is valid.
+
+If a helper is shared by schema access code and cell operation code, it should
+stay in `mod.rs` until the cell operations module is extracted. Cross-module
+call dependencies are acceptable; semantic ownership boundaries are not.