Feat/nev 64 65 66 hardening

[mneveroff]

Summary

• NEV-64: Extract share-URL sync into useLensUrlSync so LensApp only composes map, controls, and legend.
• NEV-65: Use Zustand useShallow for multi-field store picks; keep Context for harness isolation and document the Zustand / Context / local-state split in AGENTS.md.
• NEV-66: Dual-buffer map overlays with stable source IDs so year changes keep the prior paint until the replacement tiles load; legend keeps prior area totals until the next response arrives.
• Stabilize Vitest browser deps (React dedupe + zustand/react/shallow pre-bundle) so useShallow works in browser and map-harness configs.

Test plan

• Change Timeline year: map overlay should not blank to basemap-only before the new year tiles paint
• Legend totals should not flash to “None”/0 while the next area request is in flight
• Split/Swipe: change comparison year and confirm the same stability
• Share URL / browser history still updates for mode, dataset, years, and view
• pnpm test (node + browser) and map-harness suite still pass with the Vitest dep tweaks