refactor(ui): extract parseRoute and add ViewScope with useView (RIG-4439)

[rigel-mintaka]

This PR is part of a stack containing 7 PRs:

1. main
2. "refactor(ui): extract parseRoute and add ViewScope with useView (RIG-4439)" (this PR)
3. refactor(ui): move routed surfaces and agent-workspace state onto the view scope (RIG-4442) #1675
4. feat(ui): window layout model, hash sync and session persistence (RIG-4443) #1743
5. feat(ui): view tab strip with keep-alive and open-in-tab links (RIG-4444) #1755
6. feat(ui): W leader chords and palette commands for tabs and panes (RIG-4445) #1770
7. feat(ui): two-pane split with a resizable splitter (RIG-4446) #1780
8. fix(ui): chrome cleanup: drop ADE and usage bar, Session Log naming, pane edges, row re-measure (RIG-4771) #1811

T1 of the in-window tabs and split views record (docs/designs/ui/compass-in-window-tabs/design.md §T1). No user-visible change; no surface reads useView() yet (T2).

• view-route.ts: RouteMatch, pure parseRoute (the parse applyRoute did inline; unknown path → Bridge) and its inverse routePath.
• store.ts: applyRoute is now a switch over parseRoute. firstChannelId moves to module scope and takes channels, so the scope reuses it.
• view-scope.ts: ViewScope, createViewScope, ViewContext, useView(). The pending-aware hold-then-fallback runs inside the scope and navigates only that view.

One behavior difference: on main, an unknown path left the routed state untouched until the router's * redirect to / landed. parseRoute maps it to the Bridge directly, which is where that redirect goes.

The store's applyChannelRoute/applyTopicRoute tests in routing.test.tsx stay. Production still runs that path until T2 moves surfaces onto the scope.

Tests

• moon run compass-ui:test: 1212 pass, 0 fail. compass-ui:typecheck and stylelint pass; biome clean on the new files.
• New cases (all pass): 7 parseRoute/routePath round-trips plus unknown→Bridge; 4 ViewScope cases for channel and topic hold-then-resolve and hold-then-fallback, each also asserting a second scope stays put; useView throws outside a provider.
• Red check: with the scope's fallback disabled, both fallback cases fail.

Spec-impact: none.
Ledger-impact: none

Co-authored-by: Matt Wilkinson matt@rigel.build

[trunk-io]

😎 This pull request was merged.

[linear-code]

RIG-4439

[github-actions]

Compass engineering docs preview: https://compass-ui-rig-4439-view-sco.compass-eng-docs.pages.dev

Deployed from compass-ui/rig-4439-view-scope at 0cb4119.

[trunk-io]

This pull request is queued for merge as part of 1798, which will merge 1652, 1675, 1743, 1755, 1770, 1780, 1787, 1798.

[trunk-io]

This pull request was merged into main as part of stacked PR 1798.