feat(viewer): post/surface vocabulary — engine UI, host contract, helper rename (A5)

[benvinegar]

What

A5 of the workspace ▸ session ▸ post ▸ surface rename: adopt the post/surface vocabulary throughout the viewer engine, the host contract, and one server helper. A post is the published artifact (an ordered list of surfaces); a surface is one block inside a post.

Builds on #135 (model/store types) and #140 (wire routes + MCP tools). Types were already correct; this is the UI / identifier / label / CSS / host-contract sweep.

Behavior is unchanged — wire strings kept byte-identical

This is an internal rename. Every string that crosses to the server or to embedders is preserved verbatim, so self-hosted behavior is identical:

• /api/* fetch paths (incl. /api/surfaces/...), /s/ nav URLs, ?part= query key
• SSE event types surface-created / surface-updated / surface-deleted
• comment body { surface: <id> } key and author:"surface" value
• server-provided surfaceCount field; comment fields postId/postTitle
• the Route.surfaceId field (only the identity slug changes — see below)

Renamed functions that still hit a legacy wire path (e.g. postLink() → /s/<id>) carry an inline "legacy wire alias" comment.

Changes

• Viewer identifiers / props / signals: artifact surface→post, block part→surface (focusPost, standalonePost, posts, postLink, postImageLink, Card/StandaloneView prop post, surfaceFrames).
• Component files renamed: ImagePart→ImageSurface, JsonPart→JsonSurface, TracePart→TraceSurface (prop part→surface).
• CSS classes: .part-unsupported→.surface-unsupported, .imagepart→.image-surface, .tracepart→.trace-surface, .jsonpart→.json-surface, .tl-surface→.tl-post (emitters updated in lockstep).
• User-visible strings: "surface"→"post"; onboarding copy reworded off the retired board/snippet terms.
• Server helper: surfaceParts.ts→postSurfaces.ts, coerceSurfaceParts→coerceSurfaces, validateSurfaceParts→validateSurfaces; all importers + the coerceParts public alias in mcpHttp.ts updated.

⚠️ Host-contract change (embedders must update)

The host identity key identity.accountSlug is renamed to identity.workspaceSlug in viewer/src/host.ts and viewer/embed.d.ts. Any embedder must rename it when bumping to a release containing this PR.

sideshow-cloud follow-up: the cloud's web/host.ts has a TODO(A5) that currently feeds accountSlug: from identity.workspaceSlug. When the cloud bumps its sideshow pin to include this rename, that line must change to workspaceSlug: identity.workspaceSlug in the same change — TypeScript won't catch the nested-identity mismatch.

Review

Writer + fresh-context reviewer + adversarial (Sonnet) pass. Both reviewers verified: no wire string accidentally renamed, no order-trap collisions, all CSS↔JSX emitter pairs matched both directions, all renamed-component call sites pass the new prop, host contract correct in both files.

Gates

typecheck ✓ · test 256/256 ✓ · lint ✓ · format:check ✓ · build ✓ · changeset: sideshow minor

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