One sign-in for decePubClient: the root JWT exchanged for a persona's token

The first-party client signs in on /clientapi and exchanges that JWT on
/oauth/token (RFC 8693, subject_token_type jwt, avatar_id) for one
persona's Mastodon token. Only the seeded public application `decepub`
holds the grant; RootJwtSubjectToken validates the JWT through RootJwt,
which JwtBearer now shares (signature, lifetime, ban, deletion, session
stamp). The issued token names the avatar, never the root.

Owner decision recorded in ROADMAP; it supersedes "moving decePubClient
onto the Mastodon API is out of scope".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

CLAUDE.md

@@ -1,4 +1,4 @@
-# CLAUDE.md
+# CLAUDE.md
 
 Guidance for working in this repository. `docs/ROADMAP.md` holds the owner's decisions and the phased plan to full
 ActivityPub interop; read it before changing anything federation-, privacy- or API-shaped. `docs/INTEROP.md` is the
@@ -29,8 +29,8 @@ Privacy features in the model:
 
 The intended client is the Mastodon client API (Tusky, Elk, Phanpy, Ivory), where each avatar logs in as its own
 account. The private `/clientapi` covers what Mastodon can't express. `thepra/decePubClient`
-(https://decepub.thepra.dev) is the owner's own Pleroma-FE-like PWA. It still shows mock data and is not wired to the
-server.
+(https://decepub.thepra.dev) is the owner's own Pleroma-FE-like PWA. It signs in once on `/clientapi` and exchanges that
+JWT for each persona's Mastodon token (`PersonaExchange`), then uses both APIs.
 
 ## Route names are deliberate
 
@@ -221,6 +221,12 @@ group www-data and reaches the private mongod; `sudo -u www-data` works too.
 
 1. **A token is one persona.** Its subject is the avatar id; the root id lives only in the fifteen-minute `/oauth`
    cookie used while choosing the persona, and never in a token, an authorization or a response.
+   - **The first-party client exchanges the root JWT for one persona's token** (RFC 8693 on `/oauth/token`,
+     `subject_token_type` `urn:ietf:params:oauth:token-type:jwt`, `avatar_id`; `Api/Mastodon/Auth/PersonaExchange.cs`).
+     Only the seeded public application (`ClientApi:FirstPartyClientId`, default `decepub`) holds the grant.
+     `RootJwtSubjectToken` validates the JWT through `RootJwt`, the same rules as JwtBearer (signature, lifetime, ban,
+     deletion, session stamp). The issued token is the same kind as the authorization code flow's, so revocation and
+     `RootSessions` cover it.
 2. `/api/*` authenticates with OpenIddict validation, everything else with the old JWT (`PrivaPub` policy scheme).
    Every `/api` request re-checks that the persona's root is neither banned nor deleted (`MastodonController`).
 3. Read parameters through `Params` (query, form and JSON merged Rails-style), never MVC binding. A value type read