Function cache_lookup_phase 

pub fn cache_lookup_phase<'ws, F, S, O>(
    ctx: &RunContext<'ws, F, S, O>,
    task: &TaskId,
    predecessor_streams: &BTreeMap<TaskId, PredecessorStreamHashes>,
) -> Result<TaskLookup<'ws>, RunTaskError>
where
    F: WritableFilesystem,
    S: ProcessSpawner,
    O: RunObserver,

EXEC-007 step 1 of the single-task lifecycle: resolve the bearing project + task, derive the cache key, and consult the cache. No mutex hold is taken; no observer event fires.

Returns a TaskLookup the caller drives forward into either restore_from_hit (manifest present) or run_fresh (manifest absent). The scheduler interposes its EXEC-006 condition-3 mutex check between this function and run_fresh.

Errors

• RunTaskError::BuildKeyFailed when the task is missing from the workspace, or when cache-key derivation fails for any reason (input pattern resolution, file read, etc.).