Implement plan_catch_up from its specification

plan_catch_up in src/cadence/catchup.py is declared, documented and tested,

but its body raises NotImplementedError. Write it.

The full contract — what a window contains, when an occurrence is already taken

care of, when it cannot be planned yet, how a skip propagates, what the cap

means, and in which order the plan has to come back — is in the module's

docstring. Read it before writing a line of code; it is the specification, and

the evaluation measures against it.

The short version

• The window is [start, end), like every other window in this package. An

occurrence the time zone skipped does not exist.

• A run in history at that exact logical time settles the occurrence when it

succeeded or is still in flight; a failed run does not — the operator asked

for the window and did not get it.

• A dependency that cannot be satisfied is a skip with a reason, not a

silent omission, and a skip blocks what waits on it.

• The plan comes back in an order the scheduler could execute top-down: oldest

first, and a waiting task never before the task it waits for.

Boundaries

• Implement it in src/cadence/catchup.py; the public API of cadence stays as

it is, and nothing else in the package changes.

• The planner is pure: no clock, no log, no mutation of the DAG or the history.

Calling it twice on the same input returns the same plan.

• Do not add third-party dependencies — the standard library is enough.
• The repository's own test suite (pytest) has to stay green.