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/build_layout_css/horizontal-2_default.yaml

@@ -0,0 +1,31 @@
+input:
+  layout_preset: horizontal-2
+  zones_data:
+  - position: top
+    template_id: MOCK_top
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  - position: bottom
+    template_id: MOCK_bottom
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  override_zone_geometries: null
+expected_layout_css:
+  areas: '"top" "bottom"'
+  cols: 1fr
+  rows: 286px 285px
+  heights_px:
+  - 286
+  - 285
+  widths_px:
+  - 1180
+  ratios:
+  - 0.489
+  - 0.487
+  width_ratios:
+  - 1.0
+  computation: min_height_first + content_weight_distribution
+  dynamic_rows: true
+  dynamic_cols: false

tests/phase_z2/fixtures/build_layout_css/horizontal-2_override.yaml

@@ -0,0 +1,41 @@
+input:
+  layout_preset: horizontal-2
+  zones_data:
+  - position: top
+    template_id: MOCK_top
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  - position: bottom
+    template_id: MOCK_bottom
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  override_zone_geometries:
+    top:
+      x: 0
+      y: 0
+      w: 1.0
+      h: 0.3
+    bottom:
+      x: 0
+      y: 0.3
+      w: 1.0
+      h: 0.7
+expected_layout_css:
+  areas: '"top" "bottom"'
+  cols: 1fr
+  rows: 176px 410px
+  heights_px:
+  - 176
+  - 410
+  widths_px:
+  - 1180
+  ratios:
+  - 0.3
+  - 0.7
+  width_ratios:
+  - 1.0
+  computation: user_override_geometry
+  dynamic_rows: true
+  dynamic_cols: false

tests/phase_z2/fixtures/build_layout_css/horizontal-2_weighted.yaml

@@ -0,0 +1,31 @@
+input:
+  layout_preset: horizontal-2
+  zones_data:
+  - position: top
+    template_id: MOCK_top
+    content_weight:
+      score: 0.8
+    min_height_px: 200
+  - position: bottom
+    template_id: MOCK_bottom
+    content_weight:
+      score: 0.2
+    min_height_px: 150
+  override_zone_geometries: null
+expected_layout_css:
+  areas: '"top" "bottom"'
+  cols: 1fr
+  rows: 377px 194px
+  heights_px:
+  - 377
+  - 194
+  widths_px:
+  - 1180
+  ratios:
+  - 0.644
+  - 0.332
+  width_ratios:
+  - 1.0
+  computation: min_height_first + content_weight_distribution
+  dynamic_rows: true
+  dynamic_cols: false

tests/phase_z2/fixtures/build_layout_css/vertical-2_default.yaml

@@ -0,0 +1,31 @@
+input:
+  layout_preset: vertical-2
+  zones_data:
+  - position: left
+    template_id: MOCK_left
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  - position: right
+    template_id: MOCK_right
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  override_zone_geometries: null
+expected_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
+  computation: content_weight_distribution_cols
+  dynamic_rows: false
+  dynamic_cols: true

tests/phase_z2/fixtures/build_layout_css/vertical-2_override.yaml

@@ -0,0 +1,41 @@
+input:
+  layout_preset: vertical-2
+  zones_data:
+  - position: left
+    template_id: MOCK_left
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  - position: right
+    template_id: MOCK_right
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  override_zone_geometries:
+    left:
+      x: 0
+      y: 0
+      w: 0.4
+      h: 1.0
+    right:
+      x: 0.4
+      y: 0
+      w: 0.6
+      h: 1.0
+expected_layout_css:
+  areas: '"left right"'
+  cols: 40.0fr 60.0fr
+  rows: 1fr
+  heights_px:
+  - 585
+  widths_px:
+  - 466
+  - 700
+  ratios:
+  - 1.0
+  width_ratios:
+  - 0.4
+  - 0.6
+  computation: user_override_geometry
+  dynamic_rows: false
+  dynamic_cols: true

tests/phase_z2/fixtures/build_layout_css/vertical-2_weighted.yaml

@@ -0,0 +1,31 @@
+input:
+  layout_preset: vertical-2
+  zones_data:
+  - position: left
+    template_id: MOCK_left
+    content_weight:
+      score: 0.7
+    min_height_px: 200
+  - position: right
+    template_id: MOCK_right
+    content_weight:
+      score: 0.3
+    min_height_px: 200
+  override_zone_geometries: null
+expected_layout_css:
+  areas: '"left right"'
+  cols: 816px 350px
+  rows: 1fr
+  heights_px:
+  - 585
+  widths_px:
+  - 816
+  - 350
+  ratios:
+  - 1.0
+  width_ratios:
+  - 0.692
+  - 0.297
+  computation: content_weight_distribution_cols
+  dynamic_rows: false
+  dynamic_cols: true