Zero-alloc fast path for single-line text wrapping (Part of #1934)

[vchelaru]

Part of #1934 (runtime allocation pass). Continues the ratchet after the idle Draw pass (#3514); this one targets the full-relayout Text path the handoff comment flagged as next.

Finding

Profiling the realistic-Forms full-relayout scenario showed 1,504 of 1,584 bytes/frame (95%) came from text wrapping, from just 4 wrap calls/frame — all the same unchanged TextBox string, re-wrapped with its Width toggling ∞↔90 by the RelativeToChildren natural-size measurement (measure unconstrained, then restore). IWrappedTextExtensions.UpdateLines re-tokenized/Split/concatenated that string on every call even though it fits on one line and the result is the raw string itself.

Fix

A single-line fast path in UpdateLines: when a string has no explicit newline and already fits within the wrap width, add it as the one line and return — zero managed allocation. The word-by-word algorithm reproduces the raw string verbatim in that case (interior/leading/trailing spaces included, per #2617), so it's behavior-preserving. MeasureString is allocation-free for text without inline runs (it sums glyph advances).

Impact

Scenario	Before	After
Realistic Forms full relayout	1,584 B/f	240 B/f (−85%)
UpdateWrappedText on a fitting single line	288 B/call	0 B/call

The residual 240 B/f is the one genuinely-wrapping Text (the TextBox content re-wrapping at its constrained width) — the next ratchet target (making the word-by-word path itself allocation-free).

Tests (TDD, red→green)

• New TextTests.UpdateWrappedText_WhenTextFitsOnOneLine_DoesNotAllocate — asserted 288 → 0 (failed red before the fast path).
• Ratchet RealisticLayoutAllocationTests tightened 2,200 → 400 (pins 240).
• Full suites green: MonoGameGum.Tests 1821, Integration, SkiaGum 197, RaylibGum 454 — the ~25 existing wrapping tests (fixed-width, mid-word break, zero-width space, trailing space Crash on multiline text boxes #2617, newlines, ellipsis, inline BBCode) pin the algorithm's output.

Boyscout: removed a dead stringToUse recompute (Replace ran twice) and an unused returnString local in the same method.