fix: preserve definition locals for postponed task annotations

[tobiasraabe]

Nested tasks defined with from __future__ import annotations lost the enclosing function's local bindings before collection. Generated tasks whose product path existed only in an annotation therefore failed with NameError or could resolve against the final loop value.

Capture a separate shallow definition-local snapshot for each decorated task, including future annotations. Referenced objects remain shared; module-level future annotations continue to use current globals, and native module-level loop behavior is preserved. Release snapshots after collection and its modification hooks finish, including generated-child collection, collection-hook failures, and children registered before a generator body raises. Internal registration through the programmatic interface and after= does not retain pytask's own locals as the task's annotation namespace.

Regression tests cover per-task outputs, shallow sharing, plugin access during collection, programmatic registration, snapshot release, and pickling. Generator-body failure tests verify cleanup after RuntimeError and SystemExit, preservation of the original exception, and pickling of retained children with an unrelated generator-local lock. Six after= cases exercise global names colliding with internal locals under native and future annotations. The generator notebook asserts copied contents, and workflow filters accept slash-containing base branches.

Validation:

• 146 annotation, collection, generator, and decorator tests passed on Python 3.13 and 3.14.
• All six new after= cases failed on Python 3.14 before the fix.
• All four generator-body failure cases failed on Python 3.14 before the cleanup fix.
• just lint and just typing passed.

Follow-up work: annotation resolution when task callables are reused or later hooks revisit previously collected tasks. Broader transactional expansion and continued execution remain separate work under #985.

Follows #1025, which is now merged into main.