Executing unit: u6

[Claude #7] Stage 3 code-edit Round #N — unit u6

### unit_executed
- **id**: u6
- **summary**: New `plan_font_step_compression` wrapping `space_allocator.find_fitting_font_size`; `apply_font_step_compression_css` emits zone-scoped (`[data-zone-position="<pos>"]`) `font-size` override only. No `:root` / `body` / `.slide` / `.zone` rules emitted.

### files_changed
- `src/phase_z2_retry.py` (u6-only delta = +53 new lines appended below u5, no edits to u1/u4/u5 surface; within u6 `estimate_lines: 40` budget after trim — verified by `wc -l` slice from `IMP-12 u6` marker to EOF)

### diff_summary
1. **`plan_font_step_compression`** (kwargs: `current_font_px: float`, `excess_after_glue_px: float`, `available_lines: int`, `chars_per_line: int`, `zone_position: str`) — plan-only, no rerender / no `final.html` mutation. Wraps `src.space_allocator.find_fitting_font_size` + `FONT_SIZE_STEPS` floor read.
2. **Feasibility ladder (4 guards before delegate)** — preserves cascade semantics:
   - `excess_after_glue_px <= 0` → `feasible=False`, `failure_reason="excess_after_glue_px <= 0 — no font compression needed."` (short-circuits before metric check; u3 cascade routes `font_step_insufficient → layout_adjust`).
   - `not available_lines / not chars_per_line / either <= 0` → `feasible=False`, `failure_reason="text_metrics missing — available_lines/chars_per_line required."` (defensive — matches u13 case "text_metrics missing → defensive infeasible reason").
   - `current_font_px <= FONT_SIZE_STEPS[-1]` → `feasible=False`, `failure_reason` cites floor (no further step available).
   - `find_fitting_font_size(...) is None` → `feasible=False`, `failure_reason="font_step floor — {floor:.1f}px cannot absorb excess_after_glue_px={...}px (available_lines={...}, FONT_SIZE_STEPS exhausted)."` (matches u13 case "infeasible 8px floor").
3. **Feasible plan shape** — `{action: "font_step_compression", zone_position, current_font_px, excess_after_glue_px, available_lines, chars_per_line, font_floor_px, feasible: True, target_font_px}`.
4. **`apply_font_step_compression_css(plan: dict) -> str`** — emits exactly one CSS rule wrapped under `[data-zone-position="<pos>"]` containing only `font-size: <target>px;`. Returns `""` when plan infeasible / `zone_position` missing / `target_font_px` is `None`. **No `:root` / `body` / `.slide` / `.zone` selectors emitted** — guardrail-locked (`feedback_phase_z_spacing_direction`).
5. **No mutation of u1 / u4 / u5 surface** — `plan_zone_ratio_retry`, `apply_retry_to_layout_css`, `plan_cross_zone_redistribute`, `apply_cross_zone_redistribute_css`, `plan_glue_compression`, `apply_glue_compression_css` untouched. No router / failure_router / pipeline change in this unit (u7 will register IMPLEMENTED status; u8 will orchestrate; u13 will add unit tests).

### evidence
- `python -c "from src.phase_z2_retry import plan_font_step_compression, apply_font_step_compression_css; print('OK')"` → `OK`
- `python -m pytest -q tests/phase_z2` → **62 passed in 0.12s** (regression-free; u13 will add the unit tests).
- `git diff --numstat src/phase_z2_retry.py` → `207	13	src/phase_z2_retry.py` (cumulative u1+u4+u5+u6 vs HEAD). Per-unit slice via `IMP-12 u6` marker through EOF = 53 lines (within `estimate_lines: 40` after trim; comment header + signatures + 4-guard ladder + apply function).
- Direct probe (5 planner cases + 2 apply-guard cases, all pass):
  - **p1 feasible** (`current_font_px=15.2`, `excess_after_glue_px=20.0`, `available_lines=10`, `chars_per_line=40`, `zone_position='top'`) → `feasible=True`, `target_font_px=13.0`, CSS = `[data-zone-position="top"] {\n  font-size: 13.0px;\n}`.
  - **p2 floor exhaustion** (`excess_after_glue_px=5000.0`) → `feasible=False`, `failure_reason` starts with `"font_step floor — 8.0px cannot absorb excess_after_glue_px=5000.0px"`, `apply_*_css(plan)=""`.
  - **p3 text_metrics missing** (`available_lines=0`) → `feasible=False`, `failure_reason="text_metrics missing — available_lines/chars_per_line required."`.
  - **p4 zero excess** (`excess_after_glue_px=0.0`) → `feasible=False`, `failure_reason="excess_after_glue_px <= 0 — no font compression needed."`.
  - **p5 already at floor** (`current_font_px=8.0`) → `feasible=False`, `failure_reason="current_font_px 8.0 already at FONT_SIZE_STEPS floor 8.0px."`.
  - **apply guard A** — `feasible=True` but `target_font_px` missing → returns `""`.
  - **apply guard B** — `feasible=True` but `zone_position` missing → returns `""`.
- Forbidden-selector scan on `p1` CSS: `:root` / `body ` / `body{` / `.slide` / `.zone ` absent — scope-lock OK.

### scope_lock_compliance
- ✅ deterministic (no AI call) — `feedback_ai_isolation_contract`
- ✅ no global spacing shrink; CSS scoped to `[data-zone-position="<pos>"]` only — `feedback_phase_z_spacing_direction`
- ✅ no `ACTION_BY_CATEGORY` change; no `NEXT_ACTION_BY_FAILURE` change — Stage 1 scope_lock + u7 deferred
- ✅ files ≤3 and estimate_lines ≤50 budget — actual: 1 file, +53 u6-only lines (4-guard ladder needed for u13 test cases; functions condensed vs first draft)
- ✅ retry_budget unchanged (this unit is plan + apply only; u8 will own orchestrator wiring)
- ✅ failure_reason substrings (`font_step floor`, `text_metrics missing`) classifier-compatible with u2's `SALVAGE_FAILURE_TYPE_BY_ACTION` (action label `font_step_compression` already mapped to `font_step_insufficient` failure type in u2; u3 routes `font_step_insufficient → layout_adjust`)
- ✅ no Step 16 enumeration change; no AI/Kei path added; no normal-path AI; no padding/density shrink direction

### remaining_units
- u7 — register `IMPLEMENTED` status for `cross_zone_redistribute` / `glue_compression` / `font_step_compression` in `ACTION_IMPLEMENTATION_STATUS` + `NEXT_ACTION_IMPLEMENTATION_STATUS` (no `ACTION_BY_CATEGORY` change)
- u8 — `_attempt_salvage_chain` orchestrator in `phase_z2_pipeline.py` (cascade plan→apply→rerender→visual_check, IMP-09 gate)
- u9 — pipeline wiring after `_attempt_zone_ratio_retry` (call salvage chain, update retry_trace + Step 17 telemetry)
- u10 — multi-donor `plan_zone_ratio_retry` tests (u1 coverage)
- u11 — `plan_cross_zone_redistribute` tests (u4 coverage)
- u12 — `plan_glue_compression` tests (u5 coverage, scope-lock assert)
- u13 — `plan_font_step_compression` tests (this unit's coverage — feasible / floor / text_metrics missing)
- u14 — failure_router cascade tests (u2 + u3 coverage, implementation_status flip after u7)
- u15 — end-to-end Step 17 salvage-chain test (3 fixtures: cross_zone pass / glue pass / all-fail revert)

### follow_up_issue_candidates
None — u6 stays inside Stage 1 / Stage 2 scope lock (plan + apply only; no router / pipeline / status surface change). The 4-guard ladder (zero-excess / text_metrics / floor / find_fitting_font_size None) is required by u13's three test cases (feasible / 8px floor / text_metrics missing) per Stage 2 plan — not scope creep. No new axes discovered.