fix(dom): measureTextNode returns natural in Undefined mode (A1+A5 / #51 #58)

[re-marked]

Closes #51 (A1) and #58 (A5).

Summary

measureTextNode (the yoga measureFunc bound to every ink-text node) used to call wrapText even when yoga asked for intrinsic size — i.e. widthMode === Undefined with width = NaN. With 'truncate' it appended an ellipsis to the natural text and reported natural + 1; with 'truncate-start' it collapsed the entire string to a single "…" cell. That bogus width became the flex basis and cascaded into all kinds of downstream weirdness (silent truncation, surprise wrapping, layout assumptions that don't hold).

The fix is a single early return: in Undefined mode, skip wrap/truncate entirely and report the natural dimensions. Wrap and truncate are about fitting within an ACTUAL constraint; without one, the natural size is the only honest answer.

Why this is the right level

• Yoga's basis pass calls measureFunc with Undefined when the parent's main-axis size is unknown (typical for column parents auto-sizing to content). The contract for that call is "what's your intrinsic size?" — the prior code answered with a constrained truncation, which is a category error.
• The pre-existing if (text.includes('\n') && widthMode === Undefined) return natural branch was the same invariant for a narrower case (multiline content). It's now subsumed by the general rule.
• Wrap/truncate at render time (render-node-to-output.ts) already engages when widestLine(plainText) > maxWidth — the truth source for fit-or-overflow stays there. Nothing changes about how text is displayed once layout has a real constraint.

Test plan

• dom.test.ts — 9 unit tests of the Undefined / AtMost / Exactly contract, including the A1 ellipsis-poisoning bug and the truncate-start "whole string → '…'" pathology.
• render-node-to-output.test.ts — 8 nested-text integration tests covering the issue Nested <Text> inside flex-row parent silently truncates trailing content even with abundant slack #51 repro (row parent, abundant slack), narrower widths down to exact fit, the column-parent A1 path (where the Undefined-mode call actually fires), the truncate-start render path, and a cross-stretch column scenario.
• Full test suite: 789 passed (was 772, +17 across the new files; no regressions).
• typecheck / lint / build clean.

Commits

1. 40d97c4 test(render): pin nested-text + flex-row good-path behavior (Nested <Text> inside flex-row parent silently truncates trailing content even with abundant slack #51)
2. a412ce1 test(dom): failing tests for measureTextNode Undefined-mode invariant (Nested <Text> inside flex-row parent silently truncates trailing content even with abundant slack #51, A5: <Text> always carries flexShrink: 1 on wrap-family modes #58)
3. 9224140 fix(dom): measureTextNode returns natural in Undefined mode (closes Nested <Text> inside flex-row parent silently truncates trailing content even with abundant slack #51, A5: <Text> always carries flexShrink: 1 on wrap-family modes #58)
4. d5d8789 test(render): column-parent + cross-stretch scenarios for nested text (Nested <Text> inside flex-row parent silently truncates trailing content even with abundant slack #51, A5: <Text> always carries flexShrink: 1 on wrap-family modes #58)
5. 554b771 docs: note Undefined-mode measureTextNode invariant (Nested <Text> inside flex-row parent silently truncates trailing content even with abundant slack #51, A5: <Text> always carries flexShrink: 1 on wrap-family modes #58)

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