Add schema-first API docs drift detector (xtask check-docs)

Documentation.md's Lua API and IPC protocol sections are hand-written
prose and have drifted from the actual bread.* binding / IPC method
surface before. This adds a checked-in registry (api-schema.toml) of
every current Lua binding and IPC method, plus a new xtask crate
(`cargo run -p xtask -- check-docs`) that cross-checks it against
breadd/src/lua/mod.rs, breadd/src/ipc/mod.rs, and Documentation.md
itself, failing loudly on any mismatch. It's a drift detector, not a
doc generator — Documentation.md's prose is untouched except for one
genuine gap the bootstrap surfaced: bread.hyprland.eval had no mention
anywhere in the docs, now given a one-line example alongside its
siblings.

CONTRIBUTING.md documents the resulting contributor workflow.

Cargo.toml

@@ -4,6 +4,7 @@ members = [
     "breadd",
     "bread-cli",
     "bread-emit",
+    "xtask",
 ]
 resolver = "2"