[Feature]: [P3] Stairs render polish (top-floor overlap + shadowing)

Imported from the engineering backlog (_bmad-output/implementation-artifacts/backlog.md, story stairs-render-polish).
Priority P3 (do-first order) | Severity: low | Status: idea | Epic: UI legibility | Type: feature-request | Recorded: 2026-07-08

Problem / motivation

Ready-ish: art + small render pass. Owner observation 2026-07-08 (real-tower import): "I didn't even see the staircase." Our stairs/escalator flights read poorly: on import the owner could not spot them. Make walkways legible and nicer: (1) let the flight visually overlap slightly onto the top landing floor it connects to (as the diagonal reads in the original), rather than sitting flush; (2) add shadowing / depth (drop shadow, edge highlight) so a flight pops against the floor behind it; (3) general walkway UI polish (clearer diagonal, handrail, step shading). Cosmetic only; no engine/save impact. Complements elevator-floornum-contrast, facility-icons, and lobby-awnings.

Start with _bmad-output/implementation-artifacts/backlog.md and the stairs-render-polish story, then locate the stairs/escalator rendering entry point. Adjust only the visual treatment so flights overlap the top landing slightly and gain shadow/depth, clearer diagonals, handrails, and step shading. Confirm the change is cosmetic and does not affect engine or save behavior.