test(viewer): lock in 2 more "act of <X>" survive-cases for the chronicle inter-beat strip

[100yenadmin]

What

Adds two confirmed real-prose strings to the survive-case list in
test_real_prose_mentioning_a_beat_or_transition_survives
(viewer/tests/test_chronicle_dedup_opening.py):

• At the start of the act, the curtain rose on a darkened stage.
• The beginning of the act of treason was a single forged letter.

Both must pass through sanitizeNarration unchanged.

Why

The _BEAT_TRANSITION regex added in #890 strips inter-beat transition meta-text
("End of beat.", "Beginning the next beat.") from the player-facing chronicle. Two of
its arms (<end/close/beginning/start> of the <struct> and <begin/start> the <struct>)
could over-strip legitimate fiction containing the act of <X> idiom
(act of mercy / war / treason / contrition) — the worst-offender class, since story
quality is the project's north star.

The production fix is already shipped in #890 itself (commit a422b57): the two arms
use _STRUCT_FWD = "(?:beat|scene)" (dropping act/chapter/part) and a terminal
\s*[.!?]?\s*$ anchor. Verified under Node: all six confirmed false-positive sentences
survive the current regex, while the pre-fix form strips all six — and the engine seam
notes ("End of beat.", "Beginning the next beat.") still strip correctly.

The test file already covered 4 of the 6 confirmed strings (1 verbatim + 3 close
variants + a bonus "part of the story" case). This PR adds the remaining 2 so every
confirmed string is locked in as a regression guard. These have teeth: they fail against
the pre-fix regex, pass against the shipped one.

Test

Test-only, additive (6 lines). No production change.

cd viewer && python3 -m pytest tests/test_chronicle_dedup_opening.py -q
# 13 passed

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📥 Commits

Reviewing files that changed from the base of the PR and between 84c22c0 and 0c99581.

📒 Files selected for processing (1)

• viewer/tests/test_chronicle_dedup_opening.py

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