Directives: stats, bar-list, stacked-bar, heatmap

Part of #2 (Phase 1a).

Problem. Four primitives in the contract present quantities: stats (big-number tiles or metric cards with an optional mini bar and note), bar-list (label, bar, value rows with an optional variant marker), stacked-bar (segments with inline labels), and heatmap (a matrix with a value range and not-applicable cells). Each is used by the compliance pages (score strip, breakdown, method stats, systemic rules, rule reach, priority bar, priority mix, heatmap; brief §5) and none exists in mystmd core.

Proposed work. For each directive: options capped at what the compliance pages use (REVIEW §9's charting-DSL caveat), inline data as an item-directive body or a :file: CSV with column options, and portable output per the contract:

• stats: emits a div.dv-stats grid of div.dv-stat items; each carries value, denom, label, note, bar (0–1) and tone; falls back to a definition list.
• bar-list: emits div.dv-bars with rows carrying label, value, max, variant; falls back to a two-column table.
• stacked-bar: emits div.dv-stacked with segments carrying label, value, tone; falls back to a one-row table with a legend list.
• heatmap: emits div.dv-heatmap with row and column labels, cell values, min, max, na marker; falls back to a table.

Tone is set by the author or by a :tone-by: threshold option; the directive never encodes a domain rubric.

Acceptance criteria.

• Each directive has schema-validated fixture output and a documented option table
• Each falls back to a readable table or list in the default book theme
• Bad input (missing file, non-numeric value, unknown column) produces a fileError with the directive name and position

Start with the contract in docs/design-handoff-2026-09/README.md, especially brief §5 and REVIEW §9, then map the four directives and their compliance-page uses. Done means schema-validated fixtures, documented options, readable default-theme fallbacks, and positional fileError results for each listed bad input.