perf(text): single-space fast path in wrapLine + unit tests for #128

[chiefcll]

What

Follow-up to #128 (effective space width in wrapLine):

• Perf: reuse the precomputed spaceWidth when the separator is exactly one ' ' — the dominant case — instead of calling measureText per word gap. Multi-char runs (multiple spaces, ZWSP runs) are still measured for real. On the Canvas backend the old code paid a ctx.measureText per gap during layout.
• Tests: adds the unit coverage fix(text): improve effective space width calculation in wrapLine function #128 landed without:
  • multi-space separator counted at its real width ('aa bb' at maxWidth: 55 wraps into two lines — this was the bug fix(text): improve effective space width calculation in wrapLine function #128 fixed and previously overflowed)
  • multi-space separator preserved in output with correct width when the line fits
  • a run of multiple ZWSPs stays a zero-width separator
  • single-space separator uses the precomputed spaceWidth (asserted by passing a spaceWidth that differs from the measured value)

Why

#128 fixed a real overflow bug but merged without tests (repo rule: unit tests required) and regressed the common single-space case from a constant lookup to a measure call per gap.

Notes for reviewers

• The overflow and breakAll wrap strategies still use the raw single spaceWidth for the separator, so the multi-space underestimate persists on those paths. Pre-existing, out of scope here.
• Full unit suite passes: 331/331.

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