feat(deploy): self-host recipe, the enforced isolation flag, and the tunnel's door (P9a)

The books can live on a machine of the owner's own (plan 3.8, 8.2 P9a,
D-51). Three things make that honest instead of nominal:

- `--isolation same_user|enforced` sets `server::Isolation`, carried
  through `Served` and `BooksServer` to `books.start`. Locally the books
  stay `same_user (advisory)`; under the self-host unit the server is told
  `enforced` and reports `service_user (enforced)`. The server never
  guesses, and the literal it used to report is gone.
- `--http [ADDR]` binds the same MCP router on loopback (default
  127.0.0.1:7411) beside the Unix sockets, before the ready line, for the
  TLS proxy or tunnel in front of the server. It is the same bearer door
  and the same graceful close; a failed bind is reported and is not fatal
  (3.9). `serve` carries both flags through the host, so the system unit
  can ask for them once.

`deploy/wanderbooks-server.service` is the unit: a dedicated account, a
0700 data root, `ProtectSystem=strict`, no seccomp or Landlock (D-32), and
loopback-only networking. `deploy/README.md` is the recipe: install, setup,
move the books by `books adopt` with its head check (D-31: a copied book
is the transport), mint a bearer token, put Caddy or Tailscale Funnel in
front of the loopback door, and point Claude Code and Codex at it.

The two-account CI job (`.github/workflows/two-user.yml`) makes the
deployment and runs `crates/ledger-cli/tests/two_user.rs` as another
account: `agent_uid_cannot_read_tenant` proves the tenant root and its
journal are refused, then reads, writes, uploads, files, exports and
verifies over HTTP with the bearer (in a second book, so the moved books'
head is not advanced); `self_host_adopt_same_head` compares the moved
books' head and event count with the source clone's.
`isolation_label_is_enforced_under_the_unit` runs everywhere, including
the unit-file read that keeps the recipe from drifting.

Accounting reason for the shape: nothing here changes an entry, a report
or a figure. It changes who can reach them: the move is lossless and
verifiable at the head, and the boundary is the operating system's, not a
promise in our text. No journal code, kernel function, Kani harness or
Lean step is touched, so `just prove` is not required.

No go-ahead needed: no new journal feature or event; the live books are
unaffected.

docs/parity.md

@@ -91,3 +91,16 @@ trait), the runner (`bookspec/runner`: pass, fail or not claimed per rule),
 and the rule index from B-1 to B-45 (`bookspec/spec/rules-index.md`). Every
 existing bookspec example still runs on every `cargo test`, through both
 the loader's own checks and the `Engine` trait.
+
+### P9a self-host deployment (2026-10-26)
+
+P9a is a deployment, not a semantic feature: the same engine, store, MCP
+surface and reports on a machine of the owner's own. No row above changed
+status and the floor stands. What it adds is the enforced isolation the
+catalog's parity claims rest on in practice: a dedicated account, a data
+root only that account can read, `books.start` saying
+`service_user (enforced)` (`--isolation`), and the same bearer-guarded MCP
+door on loopback for Caddy or a tunnel. `crates/ledger-cli/tests/two_user.rs`
+proves the account boundary and the move's head in CI
+(`.github/workflows/two-user.yml`); the recipe is `deploy/README.md`.
+The cutover rule in `docs/capability-parity.md` is unchanged.