Wire config-driven chart aspect ratio and document layout sizing

Problem

dataface/core/compile/default_config.yml already declares chart.aspect_ratio, but it is not applied in the sizing or render pipeline, so behavior diverges from documented intent. Meanwhile, product CSS (for example apps/playground/static/playground.css) hardcodes aspect-ratio values such as 16 / 9, which violates the policy of keeping tunable behavior in YAML config rather than scattered literals in code. End-user docs (for example docs/docs/boards/sizing.md) state that heights follow “content or chart type aspect ratios,” which is not an accurate description of current implementation for most chart types (they use a fixed default height today). Authors and implementers need one clear story: what drives height for plot charts vs KPI vs tables, and where defaults live.

Context

• Current height policy (compile/sizing): get_chart_content_height uses a fixed DEFAULT_CHART_HEIGHT for most chart types, a fixed KPI height, and data-aware height for tables. It does not compute height from width × aspect_ratio for standard Vega-Lite charts today.
• Layout items: LayoutItem.aspect_ratio in compiled_types.py is derived after layout as calculated_width / calculated_height for responsive rendering metadata—not the same as a target aspect ratio from config.
• Related design task: discuss-authored-chart-width-height-and-aspect-ratio-controls.md covers author-level YAML overrides and Vega-Lite pass-through. This task focuses on wired defaults from global config and per-chart-type config, plus documentation accuracy. Author overrides can follow in a later task once defaults and layout rules are solid.

Goals

1. Single source of truth: All default and per-type target aspect ratios for plot-style charts live under default_config.yml (and merge with structure/theme overrides per existing config rules). No magic numeric aspect ratios in Python or product CSS for chart chrome unless documented as non-tunable presentation constants.
2. Per-chart-type defaults: Support a global default (for example chart.aspect_ratio) and overrides under chart_types.<type> (for example squarer defaults for pie / donut / arc if desired). Types that should not use aspect-based sizing (KPI, table, markdown-heavy content) must be explicitly excluded with a documented policy.
3. Consistent pipeline: When aspect applies, layout height (or equivalent) for a chart slot should be consistent with the allowed width for that item in its layout context (rows/cols/grid), subject to documented rules for same-row height in cols, min/max height from config, and interaction with user_height when set.
4. Vega-Lite alignment: Rendered spec dimensions should remain consistent with the resolved layout box (avoid reintroducing layout-vs-spec mismatch; see related discuss task).
5. End-user documentation: Update docs/docs/boards/sizing.md (and any linked guides) so they accurately describe width splitting, height rules, aspect defaults, KPI/table behavior, and optional author controls once implemented.

Non-goals (for this task)

• Full chart-level authored width / height / aspect_ratio in board YAML (tracked by the related discuss task once designed).
• Changing mdsvg image fallback aspect defaults unless required for chart layout (separate library surface).

Plan

• [x] Schema: Per-type aspect_ratio added under chart_types.<type> in default_config.yml — matches existing conventions. Global chart.aspect_ratio: 1.5 already existed.
• [x] Sizing: Extended get_chart_content_height with width kwarg; computes width / aspect_ratio from config for plot-style charts, clamped to min_height/max_height. KPI, table, spark_bar excluded via _NON_ASPECT_TYPES.
• [x] Render: Audited spec_builders.py — set_chart_dimensions already passes through the layout-resolved width/height to Vega-Lite specs with autosize: fit. No changes needed; aspect-driven heights flow through the existing pipeline.
• [x] De-hardcode: Replaced aspect-ratio: 16 / 9 in playground.css with 3 / 2 matching config. suite.css aspect-ratio values (1:1 for icons/tiles) are non-tunable presentation constants, documented as such.
• [x] Audit: Grepped for aspect literals. Playground example-thumbnail ratios (5/3, 2/3) are gallery layout, not chart config. No other chart-path aspect literals found.
• [x] Tests: 6 new tests in TestAspectRatioDrivenSizing: global default, KPI exclusion, table exclusion, per-type override, min/max clamping, cols layout integration.
• [x] Docs: Rewrote height rule in sizing.md, added Configuration table pointing to default_config.yml keys.

Implementation Progress

Changes made

1. dataface/core/compile/default_config.yml — Added aspect_ratio to chart_types: arc, pie, donut → 1.0 (square); geoshape, map, choropleth → 1.2 (slightly wide). Global chart.aspect_ratio: 1.5 was already present.

2. dataface/core/compile/sizing.py — get_chart_content_height now accepts width kwarg. For plot-style charts, computes height = width / aspect_ratio from per-type or global config, clamped to [min_height, max_height]. Added _NON_ASPECT_TYPES frozenset. Caller _get_item_content_height passes width through.

3. apps/playground/static/playground.css — Changed .chart aspect-ratio from 16 / 9 to 3 / 2 to match chart.aspect_ratio: 1.5.

4. docs/docs/boards/sizing.md — Rewrote height rule with accurate aspect-ratio description, KPI/table/spark_bar exceptions, min/max clamping. Added Configuration table.

5. tests/core/test_sizing.py — Added TestAspectRatioDrivenSizing class with 6 tests.

QA Exploration

• [x] N/A — this task changes backend sizing logic and CSS defaults. The sizing pipeline is covered by unit tests. Visual QA of rendered output is out of scope for this config-wiring task (no new UI surfaces).

Review Feedback

• [ ] Review cleared