fix: add Dapu 33+33 to 35 tone sandhi rule

[Aiuanyu]

Add the missing Dapu 33+33 -> 35 tone sandhi rule in sandhi_rules.json and a corresponding test case in tests/sandhi_fixture_test.js.

Fixes #239

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PR created automatically by Jules for task 17932127947538961153 started by @Aiuanyu

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[claude]

Review

Clean, minimal, data-only fix — nice work.

Correctness

• The new "33": "35" entry under da.33.when is correct: in reverse_tone_mapping.json, the macron vowel (ā) maps to tone 33 for the da dialect, so sān sān classifies both syllables as 33, and the rule now correctly promotes the front syllable to 35 — matching the audio evidence described in [sandhi] 大埔 33 + 33 發現未變調 #239.
• Traced through getSandhiHtml/classifyTone/applySandhiRule in main.js: the rule lookup is fully data-driven (rule.when[nextTone]), so this addition can't collide with or shadow any other dialect/tone combination. No code changes were needed, which is the right scope for this fix.

Test coverage

• The new fixture (sān sān → sandhi-t35) is placed correctly under the 大埔 (da) section and follows the existing fixture format/style exactly.
• Since this is a when-only rule (no always/else fallback for this case beyond null), it might be worth a follow-up fixture for a da 33 + some untriggering tone case to confirm else: null still leaves it unmodified — but that's arguably already covered generically by other dialects' fixtures, so not blocking.

Other

• No security, performance, or style concerns — this is a tiny, well-scoped JSON data change plus a matching test, and CI (sandhi-test.yml) will exercise it automatically.

Looks good to merge once CI passes.