feat(joinir): Phase 78 - BindingId infrastructure for promoted carriers (dev-only)

Phase 78 adds infrastructure to assign BindingIds to synthetic promoted
carriers (e.g., is_digit_pos, is_ch_match), enabling type-safe promoted
variable lookup without string-based naming conventions.

Key Changes:
1. CarrierVar.binding_id field (dev-only):
   - Added Option<BindingId> to track BindingId for each carrier
   - Updated all constructors and struct instantiations

2. CarrierBindingAssigner Box (new file, 273 lines):
   - Allocates BindingIds for promoted carriers via builder.allocate_binding_id()
   - Records original → promoted mapping in promoted_bindings
   - Sets binding_id field on promoted CarrierVar
   - Includes 3 comprehensive unit tests

3. ConditionEnv.register_carrier_binding() (new method):
   - Registers carrier BindingId → ValueId mappings
   - Enables type-safe lookup via binding_id_map

4. Logging cleanup:
   - Gated 6 eprintln! statements with NYASH_JOINIR_DEBUG
   - Unified logging tags to [binding_pilot/*]

Design Decisions:
- Promoters create CarrierInfo, lowering code assigns BindingIds
- CarrierBindingAssigner called from Pattern2/4 lowering (has builder access)
- Clear documentation prevents misuse (promoters lack builder access)

Files modified (18):
- carrier_info.rs: binding_id field added to CarrierVar
- carrier_binding_assigner.rs: New Box for BindingId allocation
- condition_env.rs: register_carrier_binding() method
- mod.rs: Module exports
- pattern2_with_break.rs, pattern4_with_continue.rs: Updated for binding_id
- loop_body_*_promoter.rs: Logging cleanup + binding_id in structs
- phase78-bindingid-promoted-carriers.md: Architecture documentation

Tests: 970/970 PASS (zero regressions)
Status: Infrastructure complete, integration deferred to Phase 79

Next Phase: Wire CarrierBindingAssigner in Pattern2/4 lowering + E2E tests

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/join_ir/lowering/mod.rs

@@ -20,6 +20,7 @@
 //! - `loop_pattern_router.rs`: Phase 33-12 Loop pattern routing (Pattern 1-4 dispatcher)
 
 pub(crate) mod bool_expr_lowerer; // Phase 168: Boolean expression lowering (unused - candidate for removal)
+pub mod carrier_binding_assigner; // Phase 78: BindingId assignment for promoted carriers (dev-only)
 pub mod carrier_info; // Phase 196: Carrier metadata for loop lowering
 pub(crate) mod carrier_update_emitter; // Phase 179: Carrier update instruction emission
 pub(crate) mod common; // Internal lowering utilities