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.

crates/ledger-host/src/opener.rs

@@ -100,6 +100,9 @@ pub struct Opener {
     opened_at: Option<std::time::Instant>,
     /// Whether this holder has held a lease (INVARIANT 5).
     has_held: bool,
+    /// Flags the server this opener starts is started with (P9a):
+    /// `--isolation` and `--http`. Empty for every ordinary holder.
+    server_flags: Vec<String>,
 }
 
 /// A held lease: the books stay open while it lives.
@@ -146,9 +149,20 @@ impl Opener {
             failures: 0,
             opened_at: None,
             has_held: false,
+            server_flags: Vec::new(),
         })
     }
 
+    /// Start the server this opener brings up with these flags (P9a:
+    /// `--isolation`, `--http`). Only `serve` uses it: on the owner's own
+    /// machine it is the one holder, so the host it starts is the server's
+    /// only one. A host already running was started with other flags, and
+    /// this one has no way to change it.
+    pub fn starting_with(mut self, flags: Vec<String>) -> Self {
+        self.server_flags = flags;
+        self
+    }
+
     /// The timings in force.
     pub fn knobs(&self) -> Knobs {
         self.knobs
@@ -256,9 +270,12 @@ impl Opener {
     async fn start(&self) -> Result<(), String> {
         let root = self.root.clone();
         let knobs = self.knobs;
-        tokio::task::spawn_blocking(move || crate::start::start_host(root.as_deref(), &knobs))
-            .await
-            .map_err(|e| e.to_string())?
+        let flags = self.server_flags.clone();
+        tokio::task::spawn_blocking(move || {
+            crate::start::start_host(root.as_deref(), &knobs, &flags)
+        })
+        .await
+        .map_err(|e| e.to_string())?
     }
 
     /// Say hello on a connection and read the host's first word.
@@ -457,6 +474,7 @@ mod tests {
             failures: 0,
             opened_at: None,
             has_held: false,
+            server_flags: Vec::new(),
         };
         for _ in 0..MAX_TRIES {
             o.failed().await.unwrap();