feat(showcase): stream packed taxi rows into Spatial Atlas

[ibgreen-openai]

Goals

• let the Billion-Point Spatial Atlas consume a real packed TaxiPointSource instead of always regenerating its deterministic fixture;
• preserve exact source provenance and literal source/GPU telemetry through query, rendering, and picking; and
• keep source activation, GPU rebuilds, LiDAR cache publication, and diagnostic readback transactional.

Changes

• Adds an optional ?taxi-manifest= Atlas source path with cached metadata, explicit CRS validation, capacity-bounded row-group loading, abort/reload handling, last-good fallback, and original-row tooltip provenance.
• Extracts shared taxi coordinate/data helpers used by both the Atlas and deck.gl adapter, avoiding a discarded local-XYZ allocation in the deck path.
• Reports manifest corpus, network/decode, resident, intersected-cell, candidate, match, render, and overflow values separately.
• Snapshots GPU query counters atomically into one five-word readback buffer and defers superseded resource destruction until an in-flight read settles.
• Makes Atlas/LiDAR resource replacement transactional, including postprocessing resize rollback and committed borrowed-buffer restoration.
• Adds a deterministic six-row packed-source browser fixture, scan/index and mode-transition coverage, reference-device-only mapped readback, and source/CRS/telemetry documentation.

Verification

• nvm use — passed (v22.22.1).
• yarn install — attempted; blocked by the internal socket registry returning HTTP 403 for Playwright 1.62.1 and @vis.gl/dev-tools 2.0.0-alpha.3.
• yarn lint fix — wrapper unavailable because the blocked install left ocular-lint missing; direct Biome check --write on every touched TS/JS source passed with no changes.
• yarn build — wrapper unavailable because ocular-clean is missing; focused strict TypeScript compilation passed and the Atlas Vite production bundle passed (2,127 modules).
• yarn test — wrapper unavailable because ocular-test is missing; the two focused taxi source/data suites passed (12/12).
• yarn website:build — attempted; stopped at missing Vite from the blocked install.
• (cd website && yarn build) — attempted; stopped at the same missing Vite dependency.
• Atlas Playwright software-WebGPU smoke — passed in 9 seconds, covering packed-source activation, query/execution controls, taxi-zone changes, zoom, LiDAR transition, colour mode, and bounded layout.
• Exact mapped counter/readback assertions remain opt-in for reference-device runs; the software smoke deliberately follows the existing test(showcase): validate Atlas EPT lifecycle #2919 policy.
• git diff --check — passed.
• Independent final code review — no actionable findings.

Stack

• Base: current master, including the merged WebGPU CI stabilizer test(webgpu): stabilize software renderer validation scopes #3002.
• Independent sibling of feat(deck): extract reusable luSpatial geographic query effect #2926. Either leaf can merge first; the remaining leaf should then be rebased onto the updated master.

Risks and follow-up

• The original Paul Taylor Arrow object still has no authoritative CRS metadata; operators must not label it OGC:CRS84 without external verification.
• Large public data and reference-GPU benchmarks remain intentionally outside CI.