docs: Phase 100 P2 mutable accumulator contract

- Add P2 section to phases/phase-100/README.md
- Document allowed form: s = s + x (x must be read-only)
- List Fail-Fast cases (reversed ops, complex RHS, multiple updates)
- Add example with numeric output

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

docs/development/current/main/phases/phase-100/README.md

@@ -81,3 +81,31 @@ loop(i < 1) {
 - **Fixture**: `apps/tests/phase100_pinned_local_receiver_min.hako`
 - **Smoke Test**: `tools/smokes/v2/profiles/integration/apps/phase100_pinned_local_receiver_vm.sh`
 - **Regression**: Phase 96 and Phase 94 smoke tests pass
+
+## P2: Mutable Accumulators (s = s + x form only)
+
+**Constraint**: Mutable variables are LIMITED to accumulator pattern.
+
+**Allowed form**:
+- `s = s + x` where x ∈ {Const, BodyLocal, Captured, Pinned, Carrier}
+
+**Fail-Fast (未対応)**:
+- Any other mutation: `s = f(...)` / `s = x + s` / `s += x` / multiple updates / conditional updates
+- Reason: Shape explosion prevention, safe accumulator support (JSON/CSV building)
+
+**Example** (now works):
+```hako
+local out = ""
+loop(i < 2) {
+    local ch = "a"
+    out = out + ch    # Accumulator form → LoopState carrier
+    i = i + 1
+}
+print(out.length())   # Output: 2
+```
+
+**Implementation**:
+- **P2-1**: MutableAccumulatorAnalyzer (AST shape detection only)
+- **P2-2**: Pattern2 wiring (ScopeManager delegates read-only check)
+- **P2-3**: Lowering (carrier update emission)
+- **P2-4**: Integration test (fixture + smoke, length-based validation)