DCE: Task 4 - Declarations use map → list → merge pattern

[cristianoc]

Applies the same architectural pattern from Task 3 (annotations) to declarations.

New modules

• Declarations.ml/.mli: builder (mutable) and t (immutable) types
• CollectAnnotations.ml/.mli: AST traversal for @dead/@live/@genType annotations

Key changes

1. Declarations pattern:

   • process_cmt_file creates local Declarations.builder
   • processCmtFiles collects file_data list (annotations + decls)
   • Declarations.merge_all combines into immutable t
   • Solver uses Declarations.t (read-only)

2. Global state removed:

   • Deleted global DeadCommon.decls
   • All declaration access now through explicit ~decls parameter

3. AST processing separation:

   • CollectAnnotations.ml: annotation traversal (~150 lines)
   • DceFileProcessing.ml: coordinator (~80 lines, was ~230)

4. Threading ~decls:builder through AST processing:

   • addDeclaration_, addValueDeclaration
   • DeadValue.processStructure, processSignatureItem, traverseStructure
   • DeadType.addDeclaration
   • DeadException.add
   • DeadOptionalArgs.addFunctionReference

Data flow

process_cmt_file (per-file)
    ├── CollectAnnotations → FileAnnotations.builder
    └── DeadValue/Type/Exception → Declarations.builder

Merge phase:
    FileAnnotations.merge_all → FileAnnotations.t
    Declarations.merge_all → Declarations.t

Solver:
    reportDead ~annotations ~decls (both immutable)

Benefits

• Order independence: files can be processed in any order
• Parallelizable: map phase can run concurrently (future)
• Incremental: replace one file's builders, re-merge (future)
• Type-safe: t types have no mutation functions