docs: Phase 100 P3 string accumulator contract

- Add P3 section to phases/phase-100/README.md
- Document allowed form: out = out + ch (Variable RHS only)
- List Fail-Fast cases (Literal RHS, MethodCall RHS, non-string-ish)
- Add example with length-based output

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

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

@@ -111,3 +111,34 @@ print(out.length())   # Output: 2
 - **P2-4**: Integration test (fixture + smoke, length-based validation)
 
 **LLVM EXE parity**: P2 accumulator を LLVM EXE smoke でも固定（phase100_mutable_accumulator_llvm_exe.sh）
+
+## P3: String Accumulator (out = out + ch, Variable RHS only)
+
+**Constraint**: String accumulator は最小形のみサポート。
+
+**Allowed form**:
+- `out = out + ch` where ch ∈ {Variable (string-ish)}
+- RHS must be Variable (not Literal, not MethodCall)
+
+**Fail-Fast (未対応)**:
+- Literal RHS: `out = out + "x"` (P3.1 で解禁予定)
+- MethodCall RHS: `out = out + substring(...)` (P3.2 で解禁予定)
+- Non-string-ish RHS: 型判定は既存の box facts に委譲
+
+**Example** (now works):
+```hako
+local out = ""
+local s = "abc"
+loop(i < 3) {
+    local ch = s.substring(i, i+1)    # body-local string 1文字
+    out = out + ch                    # String accumulator
+    print(out.length())               # Output: 1, 2, 3
+    i = i + 1
+}
+```
+
+**Implementation**:
+- **P3-1**: AccumulatorKind::{Int, String} 追加（型判定は委譲）
+- **P3-2**: StringAccumulatorEmitter 専用箱（JoinIR lowering）
+- **P3-3**: Pattern2 wiring（string carrier 昇格、emitter 分岐）
+- **P3-4**: Fixture + smokes（VM + LLVM EXE）