refactor(codegen): extract schema/field-access helpers into schema.rs

Move 44 schema/type-layout methods into src/codegen/schema.rs (~1,503
extracted lines), covering:

- Schema bounds and size checks (emit_loaded_schema_bounds_check,
  emit_loaded_schema_exact_size_check)
- Molecule table field helpers (emit_molecule_table_field_bounds_to_t5,
  emit_molecule_table_field_span_to_t5_t6)
- Cell metadata/field hash equality (emit_cell_metadata_equality,
  emit_cell_field_hash_equality)
- Fixed-field pointer loading (emit_loaded_fixed_field_pointer_to_stack,
  emit_dynamic_fixed_field_pointer_to_stack)
- Dynamic table field helpers (emit_dynamic_table_field_equality_check,
  emit_dynamic_table_field_span_to_stack,
  emit_dynamic_table_fixed_field_pointer_to_stack)
- Loaded field comparison (emit_loaded_field_equals_expected,
  emit_loaded_fixed_bytes_against_source, emit_loaded_field_bytes_equals_expected)
- Fixed-byte source/comparison (emit_fixed_byte_source_byte_to,
  emit_fixed_byte_comparison, expected_fixed_byte_source,
  emit_generic_fixed_byte_comparison, emit_store_fixed_byte_const_to_scratch)
- u128 and prelude value resolution (operand_is_u128,
  emit_u128_add_sub_with_u64, emit_prelude_u64_value_source_to_t1,
  emit_schema_field_source_to_t1, emit_expected_operand_to_t1)
- Schema field source preparation (emit_prepare_schema_field_source,
  emit_schema_field_source_pointer_to)
- Field access dispatch (emit_schema_field_access,
  emit_dynamic_schema_field_access, emit_aggregate_field_access,
  emit_tuple_call_return_field_access, emit_generic_field_access)

Also includes cargo fmt pass across existing submodules.

Visibility changes in mod.rs:
- Extracted methods: fn -> pub(crate) fn
- Types: SchemaFieldLayout, ExpectedFixedByteSource,
  SchemaFieldValueSource, PreludeU64ValueSource, PreludeU64OperandSource,
  SourcePointer -> pub(crate)
- Constants: CKB_SOURCE_INPUT, CKB_SOURCE_OUTPUT,
  CKB_CELL_FIELD_{CAPACITY,LOCK_HASH,TYPE_HASH},
  RUNTIME_SCRATCH_BUFFER_SIZE, RUNTIME_EXPR_TEMP_SLOTS -> pub(crate)
- Free function: fixed_byte_const_bytes -> pub(crate)

File sizes:
- mod.rs: 8,671 -> 7,168 lines (1,503 removed)
- schema.rs: 1,525 lines (new)
- assembler/runtime/abi/collections: fmt-only changes

All green: cargo check 0 warnings, 686/686 tests, clippy clean,
5/5 assembly snapshots unchanged.

Author: a19q3

src/codegen/abi.rs

@@ -13,8 +13,8 @@ use crate::ir::*;
 use crate::ENTRY_WITNESS_ABI_MAGIC;
 
 use super::{
-    fixed_aggregate_pointer_param_width, fixed_byte_pointer_param_width, fixed_register_width,
-    named_type_name, type_static_length, CellScriptRuntimeError, CodeGenerator,
+    fixed_aggregate_pointer_param_width, fixed_byte_pointer_param_width, fixed_register_width, named_type_name, type_static_length,
+    CellScriptRuntimeError, CodeGenerator,
 };
 pub(crate) const ENTRY_WITNESS_LABEL: &str = "_cellscript_entry";
 pub(crate) const ENTRY_WITNESS_MAGIC: &[u8; 8] = ENTRY_WITNESS_ABI_MAGIC;
@@ -89,7 +89,10 @@ pub(crate) fn call_param_abi_arg_count(param: &IrParam, needs_type_hash: bool) -
     }
     1
 }
-pub(crate) fn entry_witness_payload_layout(params: &[IrParam], runtime_bound_param_indices: &BTreeSet<usize>) -> Vec<EntryWitnessPayloadArg> {
+pub(crate) fn entry_witness_payload_layout(
+    params: &[IrParam],
+    runtime_bound_param_indices: &BTreeSet<usize>,
+) -> Vec<EntryWitnessPayloadArg> {
     params
         .iter()
         .enumerate()

src/codegen/assembler.rs

@@ -658,7 +658,13 @@ impl ParsedAssembly {
         })
     }
 
-    pub(crate) fn encode_section(&self, section: SectionKind, out: &mut Vec<u8>, layout: &SectionLayout, base_bias: usize) -> Result<()> {
+    pub(crate) fn encode_section(
+        &self,
+        section: SectionKind,
+        out: &mut Vec<u8>,
+        layout: &SectionLayout,
+        base_bias: usize,
+    ) -> Result<()> {
         let ops = match section {
             SectionKind::Text => &self.text_ops,
             SectionKind::Rodata => &self.rodata_ops,
@@ -1895,4 +1901,3 @@ fn pad_to_alignment(out: &mut Vec<u8>, align: usize) {
     let pad = padding_for(out.len(), align);
     out.resize(out.len() + pad, 0);
 }
-

src/codegen/collections.rs

@@ -8,9 +8,9 @@ use crate::error::Result;
 use crate::ir::*;
 
 use super::{
-    const_usize_operand, fixed_scalar_operand_width, fixed_scalar_width,
-    molecule_vector_element_fixed_width, operand_fixed_byte_width, type_static_length,
-    CellScriptRuntimeError, CodeGenerator, RUNTIME_COLLECTION_BUFFER_SIZE, RUNTIME_EXPR_TEMP_SLOTS,
+    const_usize_operand, fixed_scalar_operand_width, fixed_scalar_width, molecule_vector_element_fixed_width,
+    operand_fixed_byte_width, type_static_length, CellScriptRuntimeError, CodeGenerator, RUNTIME_COLLECTION_BUFFER_SIZE,
+    RUNTIME_EXPR_TEMP_SLOTS,
 };
 
 impl CodeGenerator {