Fix sorting of equal-time subtitles with missing indexes

[shkyyy18]

Bug

Subtitle.index supports None, and parse() accepts missing indexes. However, when two subtitles have the same start and end times, Subtitle.__lt__ directly compares their indexes. Mixing an indexed and unindexed subtitle raises TypeError in sort_and_reindex() and the default compose() path.

For example:

import srt
text = (
    "00:00:01,000 --> 00:00:02,000\nUnindexed\n\n"
    "1\n00:00:01,000 --> 00:00:02,000\nIndexed\n\n"
)
srt.compose(srt.parse(text))

Fix

Normalize missing indexes to zero in the comparison key, matching to_srt()'s existing fallback. Preserve numeric index ordering and stable order for equal fallback keys; do not mutate the input subtitles.

Validation

• Added nine synthetic regression/control cases covering both input orders, negative/zero/absent indexes, and parse/compose round trips.
• Before: eight new cases raise TypeError; the all-None control passes.
• After: python -m pytest -q --doctest-modules — 60 passed on Python 3.12.10 (pytest 9.1.1, Hypothesis 6.168.3). Original tests alone passed on the unmodified baseline (45).
• Black 26.5.1 accepts the test file. Its check on srt.py reports one pre-existing extra blank line near the imports, identically on the unmodified baseline; left that unrelated line untouched.
• The full tox / older Python matrix was not run locally.

AI assistance was used to investigate, implement, and test this change. All examples are synthetic; no private subtitle data is included.