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.

bread-module-host/Cargo.toml

@@ -0,0 +1,29 @@
+[package]
+name = "bread-module-host"
+version = "0.7.0"
+edition = "2021"
+
+[[bin]]
+name = "bread-module-host"
+path = "src/main.rs"
+
+# Deliberately minimal dependency footprint (Workstream G): this binary is
+# itself reviewable attack surface running third-party Lua under an
+# OS-level sandbox constructed by breadd's parent process (see
+# breadd/src/module_host.rs) — it does not depend on landlock itself, since
+# the Landlock ruleset is applied by breadd via Command::pre_exec() *before*
+# this binary's own main() ever runs (landlock_restrict_self() applies to
+# the calling process across the subsequent execve()).
+[dependencies]
+bread-shared = { path = "../bread-shared" }
+serde.workspace = true
+serde_json.workspace = true
+tokio = { version = "1.40", features = ["net", "io-util", "rt", "rt-multi-thread", "time", "macros", "sync"] }
+anyhow.workspace = true
+tracing.workspace = true
+tracing-subscriber.workspace = true
+mlua = { version = "0.9", features = ["lua54", "vendored", "serialize"] }
+uuid.workspace = true
+
+[dev-dependencies]
+tempfile.workspace = true