Bake task macros

Procedural macros for bake. Most users access the attribute through #[bake::task], re-exported by the core library.

The attribute preserves the original synchronous function and generates a sibling function_task() returning a task descriptor. It also adds the descriptor to Bake's link-time registration table for Registry::discover(). For unnamed library tasks, a Rust crate name beginning with bake_ supplies a default namespace: remove the prefix and replace remaining underscores with colons. Nested modules extend that namespace; matching module prefixes are included only once. Cargo binary targets keep their module-based names.

The macro emits constant-evaluated name construction using the defining module_path!(). The generated descriptor already contains the final name; manual registration and discovery use the same name. Discovery validates task metadata and detects collisions across linked crates.

Use name = "namespace:task" to override the entire command name. A short explicit name uses only the defining module's namespace, preserving existing aliases and root commands. Use runtime = ::alias if the core dependency was renamed from bake.

Use #[bake::task(output)] when a task handles its own user-facing output. Use #[bake(input)] input: bake::Value to receive the result from the preceding task.

Tasks accept owned FromStr arguments and an optional context reference. They return a Result with a serializable successful value and a displayable error. The macro rejects async, unsafe, foreign, generic, variadic, and receiver-based signatures.

See the workspace documentation for examples.