fix(layout): guard the default tab-stop grid against a non-positive interval

[leonidkuznetsov18]

What

A DOCX whose word/settings.xml carries <w:defaultTabStop w:val="0"/> froze the main thread forever: onReady/onException never fired and the tab pegged the CPU until killed (#3944).

computeTabStops (packages/layout-engine/contracts/src/engines/tabs.ts) generates the automatic tab-stop grid with pos += defaultTabInterval toward a fixed limit, so an interval of 0 (or a negative one) never advances and the loop never terminates. Both buildTabStopsPx callers (measuring/dom and layout-bridge/remeasure) pass the document value through tabIntervalTwips ?? DEFAULT_TAB_INTERVAL_TWIPS, and ?? doesn't catch 0. The grid is pre-computed for every measured paragraph, which is why any document with text content triggers it — no tab characters required (an empty <w:p/> escapes via the simple-empty-paragraph fast path, matching the repro matrix in the issue).

Fix

Skip the default grid for a non-positive (or non-finite) interval, keeping explicit stops and the implicit indent-driven stops intact. Word writes defaultTabStop 0 when a user sets the default tab interval to 0, opens such documents fine, and simply generates no automatic tab stops — so "no grid" is the Word-faithful behavior, and it mirrors the existing guard in word-layout's own computeTabStops (packages/word-layout/src/tab-layout.ts).

Guarding this single choke point covers both callers; it is the only += interval loop over the document value in the layout engine.

Testing

• New regression tests in tabs.test.ts for interval 0 (no default grid, terminates) and a negative interval (explicit stops preserved, no default grid). Without the guard the first test hangs the runner — verified by stashing the fix: the suite ran into a 3-minute wall timeout exactly as described in the issue.
• packages/layout-engine/contracts: 492/492 pass.
• Downstream consumers unchanged for positive intervals: measuring/dom 529/529, layout-bridge 1653/1653 pass.

Fixes #3944

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[leonidkuznetsov18]

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