docs(phase-92): Phase 92 P0-1 - ConditionalStep contract (SSOT) documentation

### Changes

#### 1. skeleton_types.rs - ConditionalStep Contract Documentation
Added comprehensive SSOT contract documentation for `UpdateKind::ConditionalStep`:

**Invariants** (Fail-Fast):
1. Single Update: Updates same carrier exactly once per iteration
2. Constant Deltas: Both then_delta and else_delta are compile-time constants
3. Pure Condition: Escape condition has no side effects
4. No Reassignment: Carrier not reassigned elsewhere in loop body
5. Deterministic: Update path determined solely by escape condition

**Supported Use Cases**:
- Escape sequence handling (e.g., `if ch == '\\' { i += 2 } else { i += 1 }`)
- Conditional skip patterns (e.g., `if skip { pos += len } else { pos += 1 }`)

**Fail-Fast Conditions**:
- Multiple updates → Error
- Non-constant deltas → Error
- Side-effect conditions → Error
- Carrier reassignment → Error

**Lowering Strategy** (Phase 92 P0):
Pattern2Break handles ConditionalStep by generating if-else in JoinIR with PHI merge.

### Context

Phase 92 P0-1 complete: Contract (SSOT) established for ConditionalStep.

Next step: P0-2 - Add ConditionalStep support to Pattern2 lowerer.

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

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

src/mir/loop_canonicalizer/skeleton_types.rs

@@ -65,7 +65,46 @@ pub enum UpdateKind {
     /// Constant step (`i = i + const`)
     ConstStep { delta: i64 },
 
-    /// Conditional step with numeric deltas (`if escape { i = i + 2 } else { i = i + 1 }`)
+    /// Conditional step with numeric deltas
+    ///
+    /// # Pattern
+    ///
+    /// ```text
+    /// if escape_cond { carrier = carrier + then_delta }
+    /// else { carrier = carrier + else_delta }
+    /// ```
+    ///
+    /// # Contract (SSOT - Phase 92 P0)
+    ///
+    /// ## Invariants (MUST hold, Fail-Fast otherwise):
+    /// 1. **Single Update**: Updates the same carrier exactly once per iteration
+    /// 2. **Constant Deltas**: Both `then_delta` and `else_delta` are compile-time constants
+    /// 3. **Pure Condition**: The escape condition has no side effects
+    /// 4. **No Reassignment**: Carrier is not reassigned elsewhere in the loop body
+    /// 5. **Deterministic**: Update path is determined solely by escape condition
+    ///
+    /// ## Supported Use Cases:
+    /// - Escape sequence handling (e.g., `if ch == '\\' { i += 2 } else { i += 1 }`)
+    /// - Conditional skip patterns (e.g., `if skip { pos += len } else { pos += 1 }`)
+    ///
+    /// ## Fail-Fast Conditions:
+    /// - Multiple updates to the same carrier in one iteration → Error
+    /// - Non-constant deltas (e.g., `i += f()`) → Error
+    /// - Condition with side effects (e.g., `if mutate() { ... }`) → Error
+    /// - Carrier reassignment in body (e.g., `carrier = 0`) → Error
+    ///
+    /// # Phase 92 P0: Lowering Strategy
+    ///
+    /// Pattern2Break handles ConditionalStep by generating:
+    /// ```text
+    /// if escape_cond {
+    ///     carrier_new = carrier + then_delta
+    /// } else {
+    ///     carrier_new = carrier + else_delta
+    /// }
+    /// // PHI merge at loop header: carrier_phi = phi [init, carrier_new]
+    /// ```
+    ///
     /// Phase 91 P5b: Used for escape sequence handling and similar conditional increments
     ConditionalStep { then_delta: i64, else_delta: i64 },