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.

Documentation.md

@@ -543,6 +543,10 @@ bread.hyprland.dispatch("exec", "kitty")
 -- Set a keyword
 bread.hyprland.keyword("monitor", "HDMI-A-1, 2560x1440, 0x0, 1")
 
+-- Send a raw request to the Hyprland socket, e.g. to evaluate a config-file
+-- expression the way `hyprctl eval <expr>` does; returns the raw response string
+local result = bread.hyprland.eval("some expression")
+
 -- Query compositor state (returns deserialized Lua tables)
 local win        = bread.hyprland.active_window()
 local monitors   = bread.hyprland.monitors()