refactor(phi_core): Phase 193 - Complete loopform modularization with 4-pass architecture

Phase 193: loopform_builder.rs modularization (1,166 → 102 lines, 91% reduction)

## Module Structure Created

```
src/mir/phi_core/loopform/
├── mod.rs                            (102 lines) - Public API coordinator
├── context.rs                        (148 lines) - ValueId management
├── variable_models.rs                (101 lines) - Variable types
├── utils.rs                          (112 lines) - Utilities
├── exit_phi.rs                       (96 lines) - Exit PHI builder
├── builder_core.rs                   (411 lines) - Core builder logic
├── passes/
│   ├── mod.rs                        (67 lines) - Pass coordinator
│   ├── pass1_discovery.rs            (156 lines) - Variable discovery
│   ├── pass2_preheader.rs            (70 lines) - Preheader copies
│   ├── pass3_header_phi.rs           (106 lines) - Header PHI construction
│   └── pass4_seal.rs                 (276 lines) - PHI completion
```

## 4-Pass Architecture Explicit

### Pass 1: Variable Discovery (pass1_discovery.rs)
- Classify variables as carriers or pinned
- Allocate all ValueIds upfront
- GUARD protection for invalid ValueIds

### Pass 2: Preheader Copy (pass2_preheader.rs)
- Emit deterministic copy instructions
- Order: pinned first, carriers second

### Pass 3: Header PHI Construction (pass3_header_phi.rs)
- Generate incomplete PHI nodes
- First input: preheader_copy (known)
- Second input: latch value (unknown)

### Pass 4: PHI Sealing (pass4_seal.rs)
- Complete PHI nodes with latch values
- Separate pinned/carrier handling
- PHI optimization (same-value detection)

## Size Comparison

Before:
- loopform_builder.rs: 1,166 lines (monolithic)
- loopform_passes.rs: 133 lines (documentation stub)
- Total: 1,299 lines in 2 files

After:
- 11 focused modules: 1,645 lines total
- Main file (mod.rs): 102 lines (91% reduction)
- Largest module: builder_core (411 lines)
- Average module size: 150 lines
- 4 pass modules: 608 lines (explicit structure)

## Success Criteria Met

✅ Directory structure created with 11 focused modules
✅ 4-pass architecture explicit and clear
✅ cargo build --release succeeds
✅ Loop programs execute correctly
✅ Zero breaking changes (all APIs compatible)
✅ Module documentation comprehensive
✅ All visibility correct (pub/pub(crate) appropriate)

## Files Modified

- NEW: src/mir/phi_core/loopform/mod.rs (public API)
- NEW: src/mir/phi_core/loopform/builder_core.rs (core builder)
- NEW: src/mir/phi_core/loopform/passes/*.rs (4 pass modules)
- MOVED: loopform_*.rs → loopform/*.rs (5 files)
- DELETED: loopform_builder.rs, loopform_passes.rs
- UPDATED: phi_core/mod.rs (import structure)
- UPDATED: json_v0_bridge/lowering/loop_.rs (import path)

## Impact

- **Maintainability**: Each pass has clear responsibilities
- **Testability**: Individual passes can be tested independently
- **Documentation**: Comprehensive module and pass documentation
- **Modularity**: Clean separation of concerns
- **Readability**: No file exceeds 411 lines

Phase 1-3 modularization complete. Ready for new feature development.

src/runner/json_v0_bridge/lowering/loop_.rs

@@ -24,7 +24,7 @@ use super::super::ast::ExprV0;
 use super::super::ast::StmtV0;
 use super::{lower_stmt_list_with_vars, new_block, BridgeEnv, LoopContext};
 use crate::ast::Span;
-use crate::mir::phi_core::loopform_builder::{LoopFormBuilder, LoopFormOps};
+use crate::mir::phi_core::loopform::{LoopFormBuilder, LoopFormOps};
 use crate::mir::{BasicBlockId, MirFunction, MirInstruction, ValueId};
 use std::collections::BTreeMap;