…store-backed tracked regions (#87)

## What & why

Prepares RegionKit for "more locations" ahead of letting users pick their own regions. Two coupled shifts:

1. **Split** the ~2.5 MB monolithic `us-states.geojson` into one GeoJSON file per region, loaded **on demand** — so we only ever parse the regions we track, not the whole US at launch.
2. **Replace** the hardcoded five-case `Region` enum with a **data-driven catalog** (a bundled `regions.json` manifest + a `Region` value type), so any US state (50 + DC + PR) plus Canada/EU are available regions. The user's **tracked** regions live in the synced SwiftData store (WhereCore owns "which"; RegionKit stays the agnostic geometry/lookup engine).

Onboarding's region picker and user-chosen per-region styling are **out of scope** here (designed for, not built).

## What changed

- **RegionKit — data-driven catalog + on-demand geometry**
  - `Region` is now a `Hashable`/`Codable` value type over a stable id (`us-CA`, `canada`, …) with a well-known `.other` and conveniences (`.california`, etc.). `RegionCatalog` loads the bundled `regions.json` manifest (all/name/localizedName/geometry file, canonical order). Adding a region is now **pure data** — a manifest + geojson change generated by `Tools/generate-regions.rb`, no code.
  - `RegionAttributor(for:)` loads only the passed regions' files; `.all` covers the whole catalog, `.shared` the default four. `RegionAttributing` abstracts the engine so the app can supply a live, swappable attributor. `RegionGeometryCatalog.outlines(for:attributor:)` takes an explicit attributor.
- **WhereCore — store-backed, synced tracked regions + live attributor**
  - Tracked regions persist as **one `SDTrackedRegion` row per region** (so concurrent cross-device edits merge instead of last-write-wins), read as a `Set` defaulting to the four until the user chooses.
  - `RegionAttribution` rebuilds the attributor from the tracked set on the store's `changes()` signal (local edit or remote CloudKit import). `WhereServices.make(...)` (async) derives it from the store; the app launch and the App Intents process (`WhereServices.forIntents()`, now async) both attribute against the same synced set. `make(...)` is the **sole public assembly entry**; the synchronous `init` is `@_spi(Testing)` for tests/previews.
- **WhereUI / WhereIntents — catalog-driven**
  - `RegionStyle` is id-keyed with an isolated, disposable bespoke-override table (the seam user-chosen styling will replace). Ordering uses `Region.inCanonicalOrder(_:)` instead of scanning `Region.allCases`.
  - Siri's "pick a region" suggestions and the Spotlight index surface the **tracked** set; `RegionEntity` id resolution stays **full-catalog** (so "days in Texas" answers even when untracked).
- **Backup** — tracked regions round-trip in the archive (additive field; v1 manifests decode to `[]`); `.replace` restores the set exactly, `.merge` unions it.

## Key decisions / tradeoffs

- **Clean id scheme** (`us-CA` vs `california`): existing persisted region raw values change; data is re-exported/re-imported. The `NAME → id` map lives in `Tools/generate-regions.rb`.
- **Localization tradeoff:** region names come from the manifest (with an optional `localizationKey` override), so region names lose static string-catalog extraction — consistent with how the App Intents `RegionEntity` already resolves names at runtime.
- **Attribution priority** is the manifest's canonical order (regions are mutually exclusive at our resolution).

## Deferred (follow-ups)

- The onboarding region picker and user-chosen per-region color/emoji/symbol/tint.
- Seeding semantics (the default four apply only at zero rows; the picker will seed/replace) and materializing the implicit default before the first explicit edit.
- **Untracking a region:** the store currently *deletes* the tracked-region row, which would re-attribute that region's past GPS days to `.other` on re-aggregation (manual days, stored as region sets, are safe). A `TODO` defers the soft-delete (mark inactive + load every ever-tracked region so history stays stable) to the picker work that defines the untrack UX. Not user-reachable today — nothing calls `setTrackedRegion(false)` outside tests.

## Testing

- Full `Stuff-iOS-Tests` scheme green; `./swiftformat --lint` clean.
- New/reworked coverage: catalog + value-type (`RegionTests`), subset-only-loads (`RegionAttributorTests`), rebuild-on-`changes()` + store round-trip (`RegionAttributionTests`, `TrackedRegionStoreTests`), end-to-end `make()` wiring (`WhereServicesTests`), tracked-set entity/suggestions (`RegionEntityTests`), and backup round-trip / legacy-manifest decode (`BackupServiceTests`, `BackupCoordinatorTests`).

## Review pass

- **Self-review:** log (not swallow) tracked-region read failures; deterministic (canonical-ordered) attributor build; surface unknown stored region ids; backup round-trip (above).
- **PR review feedback:** consolidated the RegionKit bundled-data docs into the main `README.md` (removed the separate `Resources/README.md`); documented why `Region`'s `Codable` is hand-written (bare id string vs the synthesized keyed object); made the sync `WhereServices.init` `@_spi(Testing)` so `make()` is the sole public entry; and TODO'd the untrack soft-delete (see Deferred).

## Kept current with `main`

Merged `#85` (CalendarDay), `#86` (Periscope JournalKit), `#88` (backup `onImport` reconcile hook), and `#89` (streamlined AGENTS + WhereCore source regroup). Conflicts resolved: the backup `formatVersion` (kept v2 **and** the additive `trackedRegions` field); the `BackupCoordinatorTests` harness swap (kept both the tracked-region tests and `#88`'s `onImport` hook test); and `Where/AGENTS.md` (took `#89`'s streamlined Modules section, but kept the accurate manifest-backed RegionKit localization note over `#89`'s stale "static keys" wording). My changes rode the source regroup into `Backup/`, `Persistence/`, `Days/`, `DataResolution/`, etc.