fix(links): finish the /p/ permalink migration — emit canonical links everywhere, teach the viewer router to parse them

[elucid]

The bug

/p/ permalinks were half-implemented in 0.11.0. PR #140 added the /p/:id server routes and made the new publish_post MCP tool emit them — but the viewer's History-API router was never taught to parse them. The result: the server resolves a /p/ link fine (byte-identical response to /s/), the SPA shell loads… and then the client-side router matches nothing, so the user lands on the workspace instead of the post. To anyone clicking an agent-shared /p/ link, the post simply "doesn't resolve."

Reproduced against a live 0.11.0 deployment — the deployed bundle's router only contains:

match(/^\/session\/([^/]+(?:\/s\/…
match(/^\/s\/([^/]+

Meanwhile, most of the app still emitted legacy /s/ links anyway (CLI, viewer copy-link, meta tags), so the canonical shape was both broken on arrival and barely used.

The fix

Parse both, write canonical. The viewer router (viewer/src/host.ts) now parses /[sp]/:id and /session/:id/[sp]/:pid inbound, and writes the canonical /p/ shapes.

Every link emitter now produces /p/:

• bin/sideshow.js — CLI publish / update output URL
• server/mcpHttp.ts + mcp/server.ts — all MCP tool responses, including the deprecated publish_surface / publish_snippet aliases (the deprecation is about tool names/args; the returned link can be canonical). postResult() loses its seg param since there's only one answer now.
• viewer/src/api.ts — copy-link (postLink) and open-as-image (postImageLink)
• server/app.ts — <link rel="canonical"> and og:url / og:image preview tags
• workers/screenshot.ts — the PNG matcher accepts both /p/:id.png and /s/:id.png, so existing /s/ share links keep their previews

Nothing inbound changes: /s/:id, /session/:id/s/:pid, and /s/:id.png remain accepted as legacy aliases and resolve exactly as before. Card iframes still load surfaces from /s/:id?part=N (internal wire, not a user-facing link).

Tests

• New e2e regression test: goto /p/:id renders the standalone full-page post — the exact scenario that shipped broken (the suite previously only covered /s/:id, which is how this slipped through).
• Updated unit/e2e assertions to the canonical shapes, keeping explicit legacy-alias coverage (worker screenshot matcher, inbound /s/ gotos, deep-link [sp] tolerance).
• ✅ 425 unit tests, 166 e2e (chromium + webkit), tsc --noEmit, oxlint.

Changeset included (patch).