feat(viewer): export getWallHideState

[Aymericr]

Closes #572.

What

getWallHideState(wallNode, wallMesh, wallMode, cameraDir) is now exported from @pascal-app/viewer, along with the WallMode type it takes.

Why

@esteban685 asked for this in #572: a host rendering its own layers inside <Viewer>'s children seam has no way to match wall-cutout visibility today. The two available workarounds are both bad:

• re-derive the getWorldDirection(v).dot(cameraDir) < 0 facing test in host code — it silently drifts the moment the viewer's rule changes
• read the assigned material variant's transparent flag — fragile, and it conflates translucent mode with a hidden wall

The predicate was already pure: it takes every input explicitly and touches no module state beyond a scratch Vector3. So this is an export, not a refactor — WallCutout calls the same function on the same line it did before.

Changes

• packages/viewer/src/systems/wall/wall-cutout.tsx — export function getWallHideState, plus a JSDoc block recording why it is public
• packages/viewer/src/store/use-viewer.ts — extract export type WallMode = 'up' | 'cutaway' | 'down' | 'translucent' so the signature names its mode instead of accepting any string. WALL_MODES already existed as the runtime counterpart; the store fields now reference the named type.
• packages/viewer/src/index.ts — re-export getWallHideState and type WallMode
• packages/viewer/src/systems/wall/wall-hide-state.test.ts — 5 cases pinning the semantics that are now public API: up always shows, down always hides, cutaway hides the near exterior face and both-sides-interior walls, and keeps both-sides-exterior walls visible from either direction

Verification

• bun test packages/viewer/src/systems/wall/ → 24 pass, 0 fail
• bun run check → 1582 files clean
• bun run check-types → 9/9 tasks pass

No behavior change: same predicate, same call site, wider visibility.

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Note

Low Risk
Export-only surface area with typed signatures and new unit tests; wall rendering behavior is unchanged.

Overview
Public API for wall cutaway visibility so hosts that render custom layers inside <Viewer> can hide or show geometry the same way WallCutout does, without copying the camera-facing test or inferring hide state from material transparency.

getWallHideState is exported from wall-cutout (unchanged logic; JSDoc explains the intent). The wallMode argument is typed as a new exported WallMode union ('up' | 'cutaway' | 'down' | 'translucent') on the viewer store and the helper. Both symbols are re-exported from @pascal-app/viewer.

Tests in wall-hide-state.test.ts lock in up/down overrides and cutaway behavior for interior/exterior combinations.

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