[Feature]: [P3] Accessibility phase 2: per-shaft congestion hatch overlay + tier badge

> Imported from the engineering backlog (_bmad-output/implementation-artifacts/backlog.md, story a11y-shaft-congestion-overlay).
> Priority P3 (do-first order) | Status: open work (see row) | Type: feature-request | Recorded: 2026-07-15

Problem / motivation

From gdd-accessibility-2026-07-01 (deferred to phase 2; the settings override in the GDD already promises to force this overlay on when it ships). A congested-band hatch drawn on the shaft plus a small tier badge answers which shaft is choking without color reliance. Distinct from the congestion-overlay row (a perf memoization of congestion()), despite the similar name. Needs a small UX pass first.

Scope addition (full repository audit 2026-07-21, finding AUD-026)

Fold the palette affordability cue into the same color-only sweep: .pal-item.unaffordable { opacity: 0.85 } (src/styles.css:298-300) is the only unaffordability signal in the build palette, no text or ARIA state. Add aria-disabled plus a non-color cue (for example a cost tint with a title) while this overlay's non-color work is in hand, so the color-only fixes land as one pass. Audit report: _bmad-output/planning-artifacts/reviews/full-repository-audit-2026-07-21/.

Read the backlog story a11y-shaft-congestion-overlay, the gdd-accessibility-2026-07-01 reference, and the AUD-026 audit before locating the overlay entry point. Check src/styles.css:298-300 for the palette affordability cue. Done means the shaft has a non-color congestion hatch and tier badge, and unaffordable palette items expose aria-disabled plus a non-color cue.