refactor(webserver): extract SPA dist serving to the frontend-static fallback seat

The webserver's built-in static dist serving becomes a single-owner fallback
seat (registerFallback/applyIndexTaps); the SPA server moves to the new
@deepseek-ai/dsh-frontend-static plugin so the composing application owns its
dist as composition, not carrier config. distIndex leaves the webserver
schema; unclaimed fallback answers 404.

packages/host/webserver/README.md

@@ -2,11 +2,11 @@
 
 English | [中文](README.zh.md)
 
-Web HTTP and upgrade-route registration plugin (default-exported `HttpServerService`, config `{host, port, distIndex}`): a `node:http` server that listens on activation and provides `ctx.httpServer`. `register(route)` adds a named `exact`/`prefix` HTTP route; `registerUpgrade(route)` adds an upgrade route for an exact pathname. A duplicate path within either table throws because route patterns are a composition-level contract and a collision is a misconfiguration; both methods return a disposer that removes the registration. `tapIndex(transform)` adds an index.html transform applied in registration order, `port` reads the listening port (the OS-assigned value when `port` is 0), and `host` reads the configured bind host (composition-time facts other plugins adapt to, e.g. the directory-picker chooser). HTTP match order is fixed: exact over the whole table, then longest prefix, then the static dist fallback with the locked semantics: traversal outside the dist root is 403, any miss falls back to `index.html` with HTTP 200 (SPA routing), unknown extensions ship as octet-stream, and non-GET/HEAD is 405. Upgrades match exactly and unmatched connections are closed; registration order carries no request-facing semantics.
+Web HTTP and upgrade-route registration plugin (default-exported `HttpServerService`, config `{host, port}`): a `node:http` server that listens on activation and provides `ctx.httpServer`. `register(route)` adds a named `exact`/`prefix` HTTP route; `registerUpgrade(route)` adds an upgrade route for an exact pathname. A duplicate path within either table throws because route patterns are a composition-level contract and a collision is a misconfiguration; both methods return a disposer that removes the registration. `registerFallback(handler)` claims the single fallback seat answering everything no named route matches — one owner only (a second claim throws; the SPA dist server [`dsh-frontend-static`](../frontend-static/README.md) is the shipped owner), 404 while unclaimed. `tapIndex(transform)` adds an index.html transform, and `applyIndexTaps(html)` runs a body through the registered transforms in order — the fallback owner calls it on every index response. `port` reads the listening port (the OS-assigned value when `port` is 0), and `host` reads the configured bind host (composition-time facts other plugins adapt to, e.g. the directory-picker chooser). HTTP match order is fixed: exact over the whole table, then longest prefix, then the fallback seat. Upgrades match exactly and unmatched connections are closed; registration order carries no request-facing semantics.
 
-The package knows no harness concepts: the `/api` HTTP bridge and downlink WebSockets are routes owned by the connection plugin, while plugin bundles and the HMR event stream are routes owned by the modules/hmr plugins. The upgrade handler owns the protocol handshake and connection contents; the webserver only delivers the raw socket and request. `host` accepts only `127.0.0.1` (default posture) and `0.0.0.0` (deliberate network exposure); `distIndex` is an assembly fact the composing app resolves and injects, never self-resolved (dist location is workspace knowledge of the app). Web (browser) shape only — Electron loads dist over `file://` and carries fetch over an IPC bridge, not this server. This package never prints; the URL line belongs to the shell.
+The package knows no harness concepts and serves no files: the `/api` HTTP bridge and downlink WebSockets are routes owned by the connection plugin, plugin bundles and the HMR event stream are routes owned by the modules/hmr plugins, and dist serving belongs to the fallback owner. The upgrade handler owns the protocol handshake and connection contents; the webserver only delivers the raw socket and request. `host` accepts only `127.0.0.1` (default posture) and `0.0.0.0` (deliberate network exposure). Web (browser) shape only — Electron loads dist over `file://` and carries fetch over an IPC bridge, not this server. This package never prints; the URL line belongs to the shell.
 
-A listen failure (EADDRINUSE…) throws out of activation and rejects Loader composition with the bind diagnostic; the failed candidate fiber is disposed. An HTTP request whose handling throws (a malformed %-escape hitting `decodeURIComponent`, a client dropping mid-body) is answered 400 — or the socket destroyed when headers are already out — and logged as a warning; it never exits the process. An upgrade-handler exception or upgraded-socket transport error is logged as a warning and destroys its socket. Disposal starts `close()` and `closeAllConnections()`, destroys every tracked upgraded socket, and returns only after the HTTP server and those sockets have closed.
+A listen failure (EADDRINUSE…) throws out of activation and rejects Loader composition with the bind diagnostic; the failed candidate fiber is disposed. An HTTP request whose handling throws (a fallback owner's `decodeURIComponent` on a malformed %-escape, a client dropping mid-body) is answered 400 — or the socket destroyed when headers are already out — and logged as a warning; it never exits the process. An upgrade-handler exception or upgraded-socket transport error is logged as a warning and destroys its socket. Disposal starts `close()` and `closeAllConnections()`, destroys every tracked upgraded socket, and returns only after the HTTP server and those sockets have closed.
 
 In development, the client-plugin registry synchronously captures each built bundle's stat baseline before it returns, then polls those baselines and re-hashes changed content. Each rescan stages its candidate table, graph, and watch map before publishing them, so a baseline failure preserves the prior graph. An immediate rebuild therefore cannot disappear into an asynchronously established watch baseline; a rename window marks the path dirty, retains the last successful baseline, and forces a re-hash when the bundle reappears even with identical metadata.
 
@@ -21,5 +21,4 @@ None; this package neither assembles nor sends a provider request.
 ## Known Limitations and Deferred Work
 
 - **No TLS, auth, or origin policy** — binding a non-loopback address exposes the server to that network; deployment hardening (or fronting it with a real reverse proxy) is deliberately out of scope for the dev-facing v1.
-- **The starter MIME table is minimal** — extensions beyond the vite-emitted set fall back to `application/octet-stream`; extend the table when an asset class actually ships.
 - **Socket options are fixed** — config selects the bind host and port, while backlog and other socket settings remain internal until a deployment needs them.