docs: Complete generic_case_a modularization (Phase 5)

- Updated mod.rs with comprehensive module organization documentation
- Documented before/after line counts and size reductions
- Removed old generic_case_a_old.rs monolith file
- Verified all imports are clean (no unused warnings)
- Verified all visibility is correct (pub(crate) for public API)

Final statistics:
- Before: 1,056 lines in single file
- After: 1,634 lines across 7 focused modules
- Main coordinator: 93 lines (91% reduction)
- Average module size: 233 lines (manageable, focused)
- Largest module: trim.rs at 537 lines (still maintainable)

All success criteria met:
✅ All 7 modules created
✅ cargo build --release succeeds
✅ Zero breaking changes (all APIs compatible)
✅ Module documentation comprehensive
✅ All visibility correct

Ref: Phase 192 modularization effort

src/mir/join_ir/lowering/generic_case_a/mod.rs

@@ -60,11 +60,22 @@
 //! // Pattern mismatch, fallback to other lowering
 //! ```
 //!
-//! ## Module Organization
+//! ## Module Organization (Phase 192)
 //!
-//! Before modularization: 1,056 lines in single file
-//! After modularization: ~100 lines coordinator + 6 focused modules (~210 lines avg)
-//! Size reduction: **90%** for main file, improved maintainability
+//! **Before modularization**:
+//! - generic_case_a.rs: 1,056 lines monolith (all 4 patterns + helpers)
+//!
+//! **After modularization** (Phase 192 complete):
+//! - mod.rs: 93 lines (coordinator, **91% reduction**)
+//! - skip_ws.rs: 258 lines (whitespace skipping)
+//! - trim.rs: 537 lines (string trimming, largest module)
+//! - append_defs.rs: 202 lines (array concatenation)
+//! - stage1_using_resolver.rs: 228 lines (namespace resolution)
+//! - entry_builder.rs: 165 lines (helper, shared initialization)
+//! - whitespace_check.rs: 151 lines (helper, shared validation)
+//!
+//! **Total**: 1,634 lines modularized (7 focused modules)
+//! **Average module size**: 233 lines (vs. 1,056-line monolith)
 //!
 //! ## See Also
 //!