fix: key pending commit work by consumer and partition

[lesnik512]

Closes #96.

Problem

PendingCommits kept one offset-sorted list per TopicPartition. When one concurrent handler serves subscribers in different consumer groups that read the same topic, both groups' user tasks for a partition shared that list, so the ready prefix was computed across both. An in-flight task in group A held back group B's finished offsets on that partition until A's task finished. Commits stayed safe, but one group's slow handler stalled the other group's commits and their share of max_uncommitted_tasks.

Change

• Pending lists are keyed by (id(consumer), partition), the same owner identity the cancellation watermarks already use. The new OwnerKey alias names it once for both.
• extract_ready_prefixes is unchanged apart from its key type. The public surface of PendingCommits (absorb, hold, take_ready, __len__, clear_watermarks) and the shape of ReadyCommit (still one per consumer) are unchanged.
• The Ready prefix glossary entry and the README's committer paragraph now say "per consumer and partition".

Tests

Regression tests written first. These fail on main:

• test_another_consumers_unfinished_task_does_not_hold_back_a_ready_prefix (both orders)
• test_unfinished_task_still_holds_back_its_own_consumers_later_tasks
• test_another_consumer_groups_in_flight_task_does_not_delay_a_commit (committer-level, the reproduction from the issue)

test_cancelled_task_sets_a_watermark_only_for_its_own_consumer passes on main and guards watermark scoping. The four extract_ready_prefixes tests now build pending state keyed by OwnerKey.

just lint passes. just test --cov=. --cov-branch gives 232 passed, 100% branch coverage.