docs: design Huge Blue app-server inspection route

docs/design/huge-blue-app-server-inspection.md

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+# Huge Blue app-server inspection route
+
+Tracking issue: [#21332](https://github.com/manaflow-ai/cmux/issues/21332)
+
+This design is intentionally separate from the durable Run attach-ticket work in
+[#21305](https://github.com/manaflow-ai/cmux/pull/21305). Inspection answers
+whether a registered remote host can be observed. It does not create a Run,
+redeem a terminal attach ticket, or mint a terminal route credential.
+
+## Boundary
+
+The inspection request is read-only and authenticated at the existing Cloud or
+VM route boundary. It accepts a registered host identity and an optional cmux
+authenticated session identity. It never accepts a raw provider URL, an SSH
+command, a private key, a daemon token, or a provider machine id supplied by a
+caller.
+
+The implementation should expose a typed status with these states:
+
+- `disabled`: the organization or local policy has disabled remote inspection.
+- `unavailable`: the configured Huge Blue adapter or app-server link is absent,
+  unhealthy, or not supported by this build.
+- `unenrolled`: the target answers, but the caller's device is not enrolled for
+  the read-only inspection channel.
+- `admitted`: the target is reachable and the caller has an enrolled,
+  authenticated inspection session.
+
+Malformed, incomplete, or contradictory probe data must resolve to
+`unavailable` or `unenrolled`; it must never be upgraded to `admitted`.
+
+## Existing primitives to reuse
+
+For an SSH-backed registered host, use the existing `SSHHost` identity and
+`SSHService`/session registry. `SSHHost.machineID` already binds the route,
+session, state directory, and optional remote mux socket. The inspection route
+should reference that registered identity instead of copying a route string into
+its own credential store.
+
+For a Cloud/app-server-backed host, reuse the existing app-server read path and
+machine connection inspection (`cloud.machine.list` and
+`cloud.machine.connect_info`). The existing cmux-tui daemon attach bundle also
+provides the fail-closed evidence we need: daemon build/protocol, enrolled-device
+membership, and the trusted-carrier listener probe. The trusted-carrier probe
+must remain an authentication signal, not a replacement for route ownership or
+team authorization.
+
+The response may include safe capability facts such as daemon protocol,
+connection state, and enrollment state. It must omit provider credentials,
+terminal tokens, invitation material, shell commands, raw route addresses, and
+arbitrary provider metadata.
+
+## Proposed response shape
+
+```json
+{
+  "host": "ssh-… or cloud-…",
+  "session": "main",
+  "state": "admitted",
+  "reasonCode": "trusted_carrier_enrolled",
+  "retryable": false,
+  "checkedAt": "2026-10-17T00:00:00.000Z",
+  "capabilities": {
+    "readOnlyInspection": true,
+    "daemonProtocol": 2,
+    "trustedCarrier": true,
+    "enrolled": true
+  }
+}
+```
+
+The final URL and transport envelope should be selected in the implementation
+PR after the owner of the app-server route confirms whether the inspection is
+served by the Cloud app server or the authenticated VM API. This design avoids
+coupling the route to the Run API while that decision is open.
+
+## Implementation slices
+
+1. Add a pure status mapper and redaction tests for disabled, unavailable,
+   unenrolled, admitted, and malformed probe states.
+2. Add the authenticated read-only route using the existing account/team route
+   helper and registered SSH/session identity.
+3. Connect the Cloud/app-server adapter to `cloud.machine.connect_info` and the
+   trusted-carrier/enrollment probe; keep Huge Blue adapter absence explicit and
+   non-retryable.
+4. Add a client inspection view that shows the typed state and remediation,
+   without offering terminal attach or shell execution.
+
+A later PR can implement these slices independently. None of them should modify
+the Run attach-ticket redemption or retention paths.