Lazy-load Marker Detail stylesheet

[elkimek]

What changed

• Replace the eager Marker Detail stylesheet link with an ordered lazy-load anchor.
• Load and apply the stylesheet before opening marker detail, manual-entry, or custom-marker modals.
• Single-flight concurrent loads, remove failed links, and use a cache-busting retry URL.
• Keep the loader in the already-eager Marker Detail runtime adapter, avoiding another startup or first-click module request.
• Keep the stylesheet in the service-worker app shell so the first offline open still works.
• Add cold-start, first-open, failure/retry, responsive, Firefox offline, and deterministic coverage-ratchet coverage.

Why

Marker Detail presentation is not needed for the initial dashboard render. Deferring its CSS removes an unconditional startup request while preserving the first-interaction experience: the modal waits for styling and the isolated first-open contract completes within 1 second.

Measured impact

Identical cold mobile returning-user runs with browser cache and service workers disabled:

• Before: 439 requests, 1,850,068 transferred bytes, 5,612,189 decoded bytes
• After: 438 requests, 1,846,079 transferred bytes, 5,585,960 decoded bytes
• Saved: 1 request, 3,989 transferred bytes, 26,229 decoded bytes

Validation

• Quality guardrails: 11/11; largest module at the 1,168-line cap
• TypeScript typecheck and check-JS typecheck
• Architecture check: 465 modules, 0 cycles
• Static verification: 131/131
• Vitest: 487/487
• Chromium Playwright: 404/404
• Responsive matrix: 472/472
• Firefox/offline smoke: 3/3
• Combined function coverage: 67.33% (67.10% ratchet)
• Final cold-load budget: 438 requests / 1,846,079 transferred / 5,585,960 decoded