chore(tiles): drop 'v2' qualifier from tile vocabulary
Tiles are how features work — there's no v1/v2 distinction left in the
runtime since the v1 removal landed (2026-04-25). This sweep cleans up
stale 'v2' mentions everywhere they appeared:
- Comments, doc references, manifest descriptions, feature READMEs/PLANs
- Type rename: TileManifestV2 -> TileManifest
- Function renames: loadV2Tile -> loadTile, loadV2Feature -> loadTileFromEntry
- Local var: isRealV2TileLaunch -> isRealTileLaunch
- Field renames on parse result and AnnotatedTile (.v2 / .v2Manifest -> .manifest)
- ManifestVersion type: ('v1', 'v2') -> ('legacy', 'tile')
Untouched: external OAuth URLs, manifest-version-2 wire-format references
in v1-removal docs (historical), Chrome MV2 fixtures (different domain).