feat(IMP-09): PR 1 — col-axis solver + per-zone geometry mapper + retry gate

Stage 3 round 4 lock implementation: extend build_layout_css beyond
the horizontal-2-only dynamic path. Every layout_css return now
carries length-locked col-axis keys (widths_px, width_ratios,
dynamic_cols) matching the parsed css_areas grid (R rows, C cols),
so 2-D layouts (T / 2x2 in PR 2) and the unified
_compute_per_zone_geometry mapper can plug in without further
contract churn.

PR 1 scope:
  - _parse_css_areas + _parse_fr_string + _compute_per_zone_geometry
    (unified — 1-D and 2-D from the same code path)
  - compute_zone_layout_cols (vertical-2 weight-only solver)
  - _build_fr_default / _build_rows_dynamic / _build_cols_dynamic
    (populate widths_px/heights_px on every return path)
  - build_layout_css override branch keeps the warn-and-fallthrough
    legacy for unsupported presets (PR 2 promotes to strict raise)
  - retry gate in _attempt_zone_ratio_retry skips when dynamic_cols=True
    or dynamic_rows=False, with explicit retry_skipped_reason
  - Step 8 artifact gains zone_widths_px_planned /
    zone_col_ratios_planned (top-level) + zone_width_px_planned /
    zone_col_ratio_planned (per-zone)
  - debug_zones width injection via _compute_per_zone_geometry
    (replaces the legacy row-only zip)

Tests: tests/phase_z2/ — 47 new cases (parse / fr-string / cols solver /
per-zone geometry / build_layout_css contract / retry gate +
6 build_layout_css YAML fixtures + 3 retry_gate fixtures).

Verification: python -m pytest -q tests = 89 passed (was 42).
horizontal-2 grid CSS strings (areas/cols/rows) byte-identical to
legacy; only additive col-axis keys are introduced.

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

tests/phase_z2/fixtures/retry_gate/horizontal2_dynamic_rows.yaml

@@ -0,0 +1,24 @@
+case_id: horizontal2_dynamic_rows
+description: |
+  horizontal-2 layout with dynamic_rows=True must pass the IMP-09 retry
+  gate. The base trace should record retry_attempted=True (legacy
+  plan/rerender path continues). retry_skipped_reason MUST NOT contain
+  either of the IMP-09 gate skip strings.
+input_layout_css:
+  areas: '"top" "bottom"'
+  cols: 1fr
+  rows: 333px 238px
+  heights_px: [333, 238]
+  widths_px: [1180]
+  ratios: [0.569, 0.407]
+  width_ratios: [1.0]
+  dynamic_rows: true
+  dynamic_cols: false
+router_decision:
+  router_active: true
+  proposed_actions_summary: [zone_ratio_retry]
+expected_gate:
+  retry_attempted: true
+  retry_skipped_reason_excludes:
+    - "dynamic_cols"
+    - "fr_default_from_preset"

tests/phase_z2/fixtures/retry_gate/single_fr_default.yaml

@@ -0,0 +1,23 @@
+case_id: single_fr_default
+description: |
+  Any layout that fell through to fr_default_from_preset (single,
+  T-shape, 2x2 in PR 1) has neither dynamic_rows nor dynamic_cols.
+  Row-axis retry is a no-op and must be skipped by the IMP-09 gate
+  with a fr_default_from_preset skip reason.
+input_layout_css:
+  areas: '"top top" "bottom-left bottom-right"'
+  cols: 1fr 1fr
+  rows: 1fr 1fr
+  heights_px: [285, 286]
+  widths_px: [583, 583]
+  ratios: [0.487, 0.489]
+  width_ratios: [0.494, 0.494]
+  dynamic_rows: false
+  dynamic_cols: false
+router_decision:
+  router_active: true
+  proposed_actions_summary: [zone_ratio_retry]
+expected_gate:
+  retry_attempted: false
+  retry_skipped_reason_contains:
+    - "fr_default_from_preset"

tests/phase_z2/fixtures/retry_gate/vertical2_dynamic_cols.yaml

@@ -0,0 +1,24 @@
+case_id: vertical2_dynamic_cols
+description: |
+  vertical-2 layout with dynamic_cols=True must be skipped by the
+  IMP-09 retry gate before plan/rerender, because the existing
+  apply_retry_to_layout_css mutates only row-axis fields and would
+  produce a misleading trace if it ran on a column-dynamic layout.
+input_layout_css:
+  areas: '"left right"'
+  cols: 583px 583px
+  rows: 1fr
+  heights_px: [585]
+  widths_px: [583, 583]
+  ratios: [1.0]
+  width_ratios: [0.494, 0.494]
+  dynamic_rows: false
+  dynamic_cols: true
+router_decision:
+  router_active: true
+  proposed_actions_summary: [zone_ratio_retry]
+expected_gate:
+  retry_attempted: false
+  retry_skipped_reason_contains:
+    - "dynamic_cols"
+    - "IMP-09"