feat: #164 Stage B CLOSURE — turn-loop JSON + cancel_observed coverage + CLAWABLE promotion

Closes all three gaebal-gajae-identified closure criteria for #164 Stage B:

1. turn-loop runtime surface exposes cancel_observed consistently
2. cancellation path tests validate safe-to-reuse semantics
3. turn-loop promoted from OPT_OUT to CLAWABLE surface

Changes:

src/main.py:
- turn-loop accepts --output-format {text,json}
- JSON envelope includes per-turn cancel_observed + final_cancel_observed
- All turn fields exposed: prompt, output, stop_reason, cancel_observed,
  matched_commands, matched_tools
- Exit code 2 on final timeout preserved

tests/test_cli_parity_audit.py:
- CLAWABLE_SURFACES now contains 14 commands (was 13)
- Removed 'turn-loop' from OPT_OUT_SURFACES
- Parametrized --output-format test auto-validates turn-loop JSON

tests/test_cancel_observed_field.py (new, 9 tests):
- TestCancelObservedField (5 tests): field contract
  - default False
  - explicit True preserved
  - normal completion → False
  - bootstrap JSON exposes field
  - turn-loop JSON exposes per-turn field
- TestCancelObservedSafeReuseSemantics (2 tests): reuse contract
  - timeout result has cancel_observed=True when signaled
  - engine.mutable_messages not corrupted after cancelled turn
  - engine accepts fresh message after cancellation
- TestCancelObservedSchemaCompliance (2 tests): SCHEMAS.md contract
  - cancel_observed is always bool
  - final_cancel_observed convenience field present

Closure criteria validated:
- ✅ Field exposed in bootstrap JSON
- ✅ Field exposed per-turn in turn-loop JSON
- ✅ Field is always bool, never null
- ✅ Safe-to-reuse: engine can accept fresh messages after cancellation
- ✅ mutable_messages not corrupted by cancelled turn
- ✅ turn-loop promoted from OPT_OUT (14 clawable commands now)

Protocol now distinguishes at runtime:
  timeout + cancel_observed=false → infra/wedge (escalate)
  timeout + cancel_observed=true → cooperative cancellation (safe to retry)

Test results: 182 → 192 passing, +10 tests, zero regression, 3 skipped unchanged.

Closes #164 Stage B. Stage C (async-native preemption) remains future work.

tests/test_cli_parity_audit.py

@@ -59,6 +59,8 @@ CLAWABLE_SURFACES = frozenset({
     'command-graph',
     'tool-pool',
     'bootstrap-graph',
+    # Turn-loop with JSON output (#164 Stage B, #174)
+    'turn-loop',
 })
 
 # Commands explicitly exempt from --output-format requirement.
@@ -75,8 +77,6 @@ OPT_OUT_SURFACES = frozenset({
     'subsystems',         # use --limit
     'commands',           # use --query / --limit / --no-plugin-commands
     'tools',              # use --query / --limit / --simple-mode
-    # Turn-loop has structured_output flag; JSON mode is future work
-    'turn-loop',
     # Simulation/debug surfaces (not claw-orchestrated)
     'remote-mode',
     'ssh-mode',