Add Workstream G: out-of-process, Landlock-sandboxed module runtime

Closes the gap Workstream D's in-process capability scoping left open:
build_scoped_env only gated presence of bread.* bindings, but os.execute/
io.open/debug.* remained fully reachable since a module's Lua still ran
inside breadd's own process. A module that declares [[permissions]] in
bread.module.toml (including an explicit empty list) is now spawned as a
separate bread-module-host process instead, restricted by a Landlock
ruleset breadd builds from that module's granted permissions and applies
via Command::pre_exec before the child executes any Lua at all. A module
with no manifest at all keeps today's in-process, ungated behavior for
backward compatibility.

- bread-module-host: new minimal binary (mlua + tokio + serde_json) that
  connects to breadd's existing IPC socket, presents a one-time spawn
  token, and proxies bread.* calls as RPC instead of direct bindings.
- breadd/src/module_host.rs: spawn + token registry + apply_sandbox
  (Landlock ruleset construction), with unit tests that spawn a real
  child and verify denial at the OS level, not a Lua-level check.
- breadd/src/ipc/module_host_bridge.rs: the module_host.* RPC bridge
  (on/once/off/emit/after/every/cancel, fs.read/write, exec/exec_capture,
  state.get, log/warn/error, status) plus the hello handshake. Bumped
  API_VERSION to 1.6.0.
- breadd/tests/module_host_sandbox.rs: end-to-end acceptance tests going
  through a real spawned breadd + bread-module-host + IPC handshake —
  os.execute/io.open denied outside a module's granted fs.read scope, and
  kill -9 on a module-host child leaving breadd and other modules intact
  while breadd reports bread.module.crashed.
- bread-shared/src/module_host_ipc.rs: shared wire types (hello result,
  tagged event/timer push envelope) so breadd and bread-module-host can't
  drift on the handshake/push shape.

Deferred (documented in Documentation.md's Workstream G section): the
trust="in-process" opt-out, remaining bread.* namespaces over RPC
(hyprland/widget/machine/bluetooth/notify/state.watch), network
sandboxing, and a fully static build that would remove the Execute grant
Landlock's dynamic-linker requirement forces on system library dirs.

Cargo.toml

@@ -4,6 +4,7 @@ members = [
     "breadd",
     "bread-cli",
     "bread-emit",
+    "bread-module-host",
     "xtask",
 ]
 resolver = "2"
@@ -18,3 +19,14 @@ tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 dirs = "6.0"
 chrono = { version = "0.4", features = ["serde"] }
 tempfile = "3"
+# Pure-Rust bindings to the Linux Landlock LSM syscalls (landlock_create_ruleset/
+# landlock_restrict_self) — kernel 5.13+, no external bwrap/firejail binary
+# dependency. See Workstream G (bread-module-host / breadd's module_host
+# spawner) — the actual OS-level sandboxing mechanism for out-of-process
+# module execution. Verified against this repo's dev kernel (6.18) with a
+# real pre_exec()-restricted child process before adoption: filesystem reads
+# outside the granted rule set are denied at the kernel level
+# (RulesetStatus::FullyEnforced / PartiallyEnforced, EACCES on the denied
+# path), not merely a Lua-level check.
+landlock = "0.4"
+uuid = { version = "1", features = ["v4"] }