feat(IMP-12): Step 16/17 retry refinement — multi-donor + 3-stage salvage cascade

Extend Step 17 deterministic action surface so donor_slack_insufficient no longer
abort-terminates at zone_ratio_retry. AI is NOT invoked on the normal salvage path.

Source changes (4 files, scope-locked):
- src/phase_z2_retry.py — plan_zone_ratio_retry: single-primary-donor → multi-donor
  greedy aggregation (donors_used / aggregate_slack_used / aggregate_slack_available);
  new plan/apply pairs: cross_zone_redistribute (wraps fit_verifier.redistribute,
  data-role scoped CSS), glue_compression (wraps space_allocator.compute_glue_css_overrides,
  data-zone-position scoped), font_step_compression (wraps find_fitting_font_size,
  zone-scoped, defensive feasible=False on missing text_metrics).
- src/phase_z2_failure_router.py — classifier inspects salvage_steps[-1] via
  SALVAGE_FAILURE_TYPE_BY_ACTION; NEXT_ACTION_BY_FAILURE rewired into
  donor_slack_insufficient/no_donor_candidates → cross_zone_redistribute → glue
  → font_step → layout_adjust; 3 IMPLEMENTED salvage status rows added.
- src/phase_z2_router.py — ACTION_IMPLEMENTATION_STATUS registers 3 new salvage
  actions as IMPLEMENTED; ACTION_BY_CATEGORY untouched (cascade-only labels).
- src/phase_z2_pipeline.py — new _attempt_salvage_chain() iterates router
  next_proposed_action with retry_budget=1 per action; honors IMP-09 dynamic_cols
  / fr_default gate; preserves (b)-revert on all-fail; wires Step 17 telemetry
  (salvage_steps / salvage_passed).

Tests (6 new pytest modules):
- test_phase_z2_retry_multi_donor.py — single sufficient (regression), 1st
  insufficient + 2nd sufficient (multi-donor PASS), aggregate insufficient FAIL.
- test_phase_z2_cross_zone_redistribute.py — multi-role zone feasible,
  single-role zone short-circuits infeasible.
- test_phase_z2_glue_compression.py — feasible asserts emitted CSS contains
  [data-zone-position=...] selector and NO global :root/body/.slide rule.
- test_phase_z2_font_step_compression.py — 15.2 → 13 closes excess; 8px floor;
  missing text_metrics → defensive infeasible reason.
- test_phase_z2_failure_router_cascade.py — donor_slack_insufficient → cross_zone
  (impl=IMPLEMENTED); 3 new failure types → expected next actions; rerender_still_fails
  preserves frame_reselect terminus.
- test_phase_z2_step17_salvage_chain.py — end-to-end (a) cross_zone PASS promotes
  final.html, (b) cross_zone FAIL + glue PASS promotes 2nd candidate, (c) all-3
  FAIL preserves original final.html (revert).

Guardrails preserved:
- AI calls: 0 on normal path (feedback_ai_isolation_contract)
- Spacing direction: no shrink-common-margin; resolve via donor/glue/font-step
  within frame envelope (feedback_phase_z_spacing_direction)
- All CSS overrides scoped to [data-role=...] or [data-zone-position=...]
- IMP-09 dynamic_cols / fr_default gate honored in cascade
- (b)-revert preserved if all 3 salvage actions fail

Refs: gitea#12 IMP-12

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/phase_z2_router.py

@@ -56,7 +56,7 @@ ACTION_RATIONALE: dict[str, str] = {
         "위 매핑 모두 미적용 — 마지막 fallback (현재 코드는 sys.exit 으로 abort)",
 }
 
-# 각 action 의 *현재 코드* 구현 상태 (2026-04-29 기준)
+# 각 action 의 *현재 코드* 구현 상태 (2026-04-29 기준; IMP-12 u7 cascade 2026-05-18)
 # A2 단계에서 이 매핑이 *어디까지 자동 처리되고 어디서 막히는지* trace 확보용
 ACTION_IMPLEMENTATION_STATUS: dict[str, str] = {
     "zone_ratio_retry":          "IMPLEMENTED",  # A3 (2026-04-29) phase_z2_retry.plan_zone_ratio_retry + pipeline orchestration
@@ -65,6 +65,12 @@ ACTION_IMPLEMENTATION_STATUS: dict[str, str] = {
     "frame_reselect":            "PARTIAL",      # IMP-05 pre-render rank-2/3 fallback implemented; post-render rerender trace-only
     "adapter_needed":            "PARTIAL",      # composition v0.1.1 의 mapper FitError catch
     "abort":                     "IMPLEMENTED",  # sys.exit(1) — pipeline 의 현재 default
+    # IMP-12 u7 (2026-05-18): cascade-only salvage actions (no ACTION_BY_CATEGORY row;
+    # surfaced via NEXT_ACTION_BY_FAILURE in phase_z2_failure_router). plan/apply pairs
+    # implemented in phase_z2_retry; pipeline orchestrator wiring lands in u8/u9.
+    "cross_zone_redistribute":   "IMPLEMENTED",  # u4 phase_z2_retry.plan_cross_zone_redistribute + apply_cross_zone_redistribute_css
+    "glue_compression":          "IMPLEMENTED",  # u5 phase_z2_retry.plan_glue_compression + apply_glue_compression_css
+    "font_step_compression":     "IMPLEMENTED",  # u6 phase_z2_retry.plan_font_step_compression + apply_font_step_compression_css
 }