docs(schema): purge stale bbox/zoom/center SystemState-spatial refs (#266)

[IanMayo]

Summary

Closes BACKLOG #266 — the spatial-shape documentation cleanup that became actionable once #249 / spec-261 shipped.

Spec-261 unified SystemStateProperties.spatial onto a single viewport: ViewportPolygon (+ optional rotation), removing the legacy bbox: float[4] / zoom: float / center: float[2] fields. This PR purges every remaining doc that still described the old shape. Documentation + one evidence fixture only — no code, schema, or test-fixtures-under-test changed.

What changed

File	Change
shared/schemas/README.md	state.spatial variant line → ViewportPolygon + rotation
specs/237-active-storyboard-persistence/data-model.md	SystemStateTypeEnum.spatial description → (ViewportPolygon)
specs/022-system-kind-discriminator/	The spec that originally defined the bbox/zoom/center shape, updated in full: acceptance criterion, Key-Entities prose, a new "⚠ Superseded by #261" note, research rationale, data-model field table + validation rule, the Python usage example, and evidence/sample-spatial.json (rewritten to the 4-corner polygon)
BACKLOG.md	Item #266 marked complete
docs/project_notes/issues.md	Work-log entry added

Scope decision

The stale references clustered in the foundational, already-shipped spec-022. Per a scope check, the chosen approach was a full purge (clean grep), including the historical sample fixture, with a "superseded by #261" note added so readers understand why a 2024-era spec now shows the 2026 shape.

Retained intentionally (different, current concepts — not the removed SystemState shape): the storyboard Viewport type (center/zoom/bearing), STAC spatial.bbox collection extent, standard GeoJSON bbox members, and the new ViewportPolygon.zoom slot.

Verification

• ✅ Trigger greps clean — SystemStateProperties.*bbox, state_type.*spatial.*bbox, and (bbox, zoom[, center]) all return zero SystemState hits (excluding spec-261's own migration docs).
• ✅ evidence/sample-spatial.json re-validated against the live SystemState Pydantic model (viewport = 4 corners, zoom = 8.5, rotation = 0); confirmed no test references the file.
• ✅ ruff / pnpm lint / pnpm -r typecheck all green (the only lint warnings are pre-existing in apps/vscode/src/services/llmProxy.ts, untouched here).
• ℹ️ Behaviour suites (pytest / vitest / Playwright) exercise no code in this diff.

https://claude.ai/code/session_019XNKPXw6XgxjrPx1nsGrNj

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Raw spec directories changed in this PR:

• specs/022-system-kind-discriminator/
• specs/237-active-storyboard-persistence/
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