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/host.rs

@@ -244,16 +244,32 @@ struct Args {
     /// server finds the root itself, and no command line names it.
     root: Option<PathBuf>,
     knobs: Knobs,
+    /// Flags the server is started with (P9a): `--isolation`, `--http`.
+    server_flags: Vec<String>,
 }
 
 fn parse_args(prog: &str, args: impl IntoIterator<Item = String>) -> Result<Option<Args>, String> {
     let mut root = None;
     let mut knobs = Knobs::default();
-    let mut args = args.into_iter();
+    let mut server_flags: Vec<String> = Vec::new();
+    let mut args = args.into_iter().peekable();
     while let Some(arg) = args.next() {
         match arg.as_str() {
             "--root" => root = Some(PathBuf::from(args.next().ok_or("--root needs a folder")?)),
             FLAG => knobs = Knobs::parse(&args.next().ok_or("--test-knobs needs a value")?)?,
+            // Passed through to the server (P9a).
+            "--isolation" => {
+                server_flags.push(arg);
+                server_flags.push(args.next().ok_or("--isolation needs a level")?);
+            }
+            "--http" => {
+                server_flags.push(arg);
+                if let Some(v) = args.peek()
+                    && !v.starts_with('-')
+                {
+                    server_flags.push(args.next().unwrap_or_default());
+                }
+            }
             "--help" | "-h" => {
                 eprintln!("usage: {prog} [--root DIR]");
                 eprintln!("  Holds the books open while anything holds a lease on it; started by");
@@ -263,7 +279,11 @@ fn parse_args(prog: &str, args: impl IntoIterator<Item = String>) -> Result<Opti
             other => return Err(format!("unknown argument: {other}")),
         }
     }
-    Ok(Some(Args { root, knobs }))
+    Ok(Some(Args {
+        root,
+        knobs,
+        server_flags,
+    }))
 }
 
 /// `wanderbooks host`.
@@ -395,6 +415,8 @@ struct Host {
     root: Root,
     run: PathBuf,
     knobs: Knobs,
+    /// Flags the server is started with (P9a).
+    server_flags: Vec<String>,
     leases: BTreeMap<u64, Held>,
     next_lease: u64,
     ready: bool,
@@ -417,6 +439,7 @@ impl Host {
             root,
             run,
             knobs: args.knobs,
+            server_flags: args.server_flags,
             leases: BTreeMap::new(),
             next_lease: 1,
             ready: false,
@@ -655,6 +678,8 @@ impl Host {
         if self.explicit_root {
             command.arg("--root").arg(self.root.path());
         }
+        // What `serve` was told, for the server (P9a).
+        command.args(&self.server_flags);
         let d = Knobs::default();
         for (flag, now, default) in [
             ("--backup-quiet", self.knobs.backup_quiet, d.backup_quiet),