feat(llvm/phi): Phase 277 P1 - fail-fast validation for PHI strict mode

## Summary
Implemented fail-fast validation for PHI ordering and value resolution in strict mode.

## Changes

### P1-1: Strict mode for "PHI after terminator"
- File: `src/llvm_py/phi_wiring/wiring.py::ensure_phi`
- Behavior: `NYASH_LLVM_PHI_STRICT=1` → RuntimeError if PHI created after terminator
- Default: Warning only (no regression)

### P1-2: Strict mode for "fallback 0"
- File: `src/llvm_py/phi_wiring/wiring.py::wire_incomings`
- Behavior: Strict mode forbids silent fallback to 0 (2 locations)
  - Location 1: Unresolvable incoming value
  - Location 2: Type coercion failure
- Error messages point to next debug file: `llvm_builder.py::_value_at_end_i64`

### P1-3: Connect verify_phi_ordering() to execution path
- File: `src/llvm_py/builders/function_lower.py`
- Behavior: Verify PHI ordering after all instructions emitted
- Debug mode: Shows "✅ All N blocks have correct PHI ordering"
- Strict mode: Raises RuntimeError with block list if violations found

## Testing
✅ Test 1: strict=OFF - passes without errors
✅ Test 2: strict=ON - passes without errors (no violations in test fixtures)
✅ Test 3: debug mode - verify_phi_ordering() connected and running

## Scope
- LLVM harness (Python) changes only
- No new environment variables (uses existing 3 from Phase 277 P2)
- No JoinIR/Rust changes (root fix is Phase 279)
- Default behavior unchanged (strict mode opt-in)

## Next Steps
- Phase 278: Remove deprecated env var support
- Phase 279: Root fix - unify "2本のコンパイラ" pipelines

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

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

src/mir/builder/variable_context.rs

@@ -92,6 +92,28 @@ impl VariableContext {
         self.variable_map.get(name).copied()
     }
 
+    /// Require a variable's ValueId (fail-fast variant of lookup)
+    ///
+    /// Returns the ValueId if the variable exists, otherwise returns Err.
+    ///
+    /// ## Use cases:
+    /// - Pattern6/7/8: Variable extraction with clear error messages
+    /// - JoinIR lowering: Carrier/invariant resolution
+    ///
+    /// ## Example:
+    /// ```ignore
+    /// let i_init = builder.variable_ctx.require(&parts.i_var, "pattern7")?;
+    /// let s_host = builder.variable_ctx.require(&parts.s_var, "pattern7")?;
+    /// ```
+    ///
+    /// Phase 272 P0.2 Refactoring: Eliminate variable_map.get().ok_or() boilerplate
+    pub fn require(&self, name: &str, context: &str) -> Result<ValueId, String> {
+        self.variable_map
+            .get(name)
+            .copied()
+            .ok_or_else(|| format!("[{}] Variable '{}' not found in variable_map", context, name))
+    }
+
     /// Insert or update a variable's ValueId
     ///
     /// ## Important notes: