test: insertElementTree direct + builder edge-case coverage
insert-element-tree.test.ts (8 cases) — previously only covered
indirectly via handler integration tests. Pins parent_id quote-
safety pre-check, byte-exact file rollback on rejection,
post-insert verification catches silent no-op, happy-path root
insert + parent-targeted insert.
element-builders-edge-cases.test.ts (20 cases) — robustness
boundary coverage: mixed-script i18n (Arabic, emoji, hangul+han,
hiragana+latin), extreme numeric values (chart_bars 1e9 / all-
zero, stepper total=100, rating_stars overflow clamp), minimal
inputs (single-item rows, solo timeline entry), extreme label
length (badge 40 char, kbd 4-key no separator), calendar_grid
boundaries (28-day, 31-day offset 6, day 0 not marked, upper
clamps).
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