2pass: keep the phrase line, and label the gaps

[KakaruHayate]

Fixes the three problems reported in the 2PASS chain.

1. The phrase line was lost

breathe --merge wrote only phones + breath, so the first pass's texts/words
were gone before the second pass even ran — and the second pass reads only the
phones tier, so nothing could rebuild them. Two changes:

• the merge now carries texts/words through into the merged TextGrid;
• align --textgrid re-attaches that grouping to the new spans. Each token is
  mapped back to the text/word interval that contained it (by time, in the input
  TextGrid), and the tier builder aggregates by exactly that owner id — so the
  tiers come out with the original words and the new boundaries.

The mapping has to skip the empty intervals the writer pads every tier with, and
has to skip the stop symbols the aligner drops; when the shapes still disagree it
declines to guess and leaves the tiers as before.

2. Stretches no token covered stayed empty

TextGridTier gains a per-tier gap_label. align and breathe --merge fill
the phones tier with it — --fill-gaps LABEL, default SP, empty string to
keep the upstream "unlabelled" behaviour. texts/words are untouched (they
still pad with empty labels, which is what upstream's textgrid package does).

3. The mechanism, confirmed before designing the fix

SP is a stop symbol: encode_phones() drops it, so an SP inserted before
the second pass cannot survive it. AP/EP/GS are global symbols and do
survive. Filling therefore belongs at serialisation, after alignment — it is a
labelling step, not a token pushed into the model. A gap that is silence is SP,
and SP costs the alignment nothing.

Evidence

hanser_0_0.wav, three steps:

① align        12 phones + 7 SP-filled gaps, tiers texts/words/phones
② breathe      4 tiers (texts, words, phones, breath)   <- was 2
③ align        14 tokens (12 phones + 2 AP) + 8 SP gaps, 0 unlabelled
               texts: 今 天 我 们 唱 歌  (re-timed to the 2PASS alignment)

Test suite 30/30 (a new TextGrid.GapLabelIsPerTierAndDefaultsToEmpty covers the
per-tier fill).

Note

The first pass also fills its gaps with SP now, since the flag defaults to SP for
both commands. That is what makes the final annotation gap-free; pass
--fill-gaps "" to keep the old behaviour.

[coderabbitai]

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