feat(joinir): Phase 170-C-2 LoopUpdateSummary skeleton and design doc

Add UpdateKind/LoopUpdateSummary types for loop update pattern analysis.
Currently uses carrier name heuristics internally (same as 170-C-1),
but provides clean interface for future AST/MIR-based analysis.

New types:
- UpdateKind: CounterLike | AccumulationLike | Other
- CarrierUpdateInfo: name + kind pair
- LoopUpdateSummary: collection with helper methods

Helper methods:
- has_single_counter(): for StringExamination detection
- has_accumulation(): for ArrayAccumulation detection

Design doc: docs/development/current/main/phase170-c2-update-summary-design.md
- Describes LoopFeatures.update_summary integration plan
- Migration path to AST/MIR analysis

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/join_ir/lowering/mod.rs

@@ -24,6 +24,7 @@ pub mod carrier_info; // Phase 196: Carrier metadata for loop lowering
 pub mod common;
 pub mod condition_to_joinir; // Phase 169: JoinIR condition lowering helper
 pub mod loop_update_analyzer; // Phase 197: Update expression analyzer for carrier semantics
+pub mod loop_update_summary; // Phase 170-C-2: Update pattern summary for shape detection
 pub mod exit_args_resolver;
 pub mod funcscanner_append_defs;
 pub mod funcscanner_trim;