fix(transform): sort by (timestamp, duration) and add flood idempotency tests

[TimeToBuildBob]

Ports two remaining improvements from #105 that weren't included in #143.

Changes

1. Sort key: (timestamp, duration) instead of just timestamp

When multiple events share the same timestamp, shorter events now sort first. This gives a deterministic processing order for same-timestamp inputs and aligns with aw-server-rust's behavior (its sort_by_timestamp processes shorter events first when timestamps are equal).

2. Two new tests

• test_flood_idempotent: verifies that calling flood() repeatedly on its own output returns the same result — an important correctness property. Adapted from fix: refactored flood to better behavior #105.
• test_flood_unsafe_gap: verifies that overlapping events with differing data don't double-count time (validates the normalization pass added in fix(transform): normalize flood() output to remove overlapping events #143). Adapted from fix: refactored flood to better behavior #105.

Both tests pass on the current master (no other code changes needed).

aw-server-rust spec comparison

Per Erik's request, I compared the Python implementation against aw-server-rust's flood.rs. The sort key change above aligns them. One notable remaining divergence:

Positive gap, differing data: aw-server-rust does "meet in the middle" — each event extends by gap/2. The Python implementation does "longer wins" — the longer event fills the entire gap. The normalization pass from #143 means both implementations produce overlap-free output, but the gap-split point differs. Leaving this for a separate discussion since it's a more significant behaviour change.

[greptile-apps]

Greptile Summary

This PR makes flood processing deterministic for events with equal timestamps. The main changes are:

• Sort equal-timestamp events by ascending duration.
• Add a test for repeated-call idempotency.
• Add a test for avoiding double-counted overlapping time.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• Equal timestamps now have deterministic ordering without breaking overlap normalization.
• The added tests match the existing event equality and duration behavior.

Important Files Changed

Filename	Overview
aw_transform/flood.py	Adds duration as the secondary event sort key; downstream normalization remains consistent with this ordering.
tests/test_flood.py	Adds focused tests for idempotency and duration conservation across overlapping events.

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