Fix line breaking for Khmer text (issue #7218)

[kenton-r]

Problem

Spaces in Khmer text create isolated segments with breaks before AND after each space:

• Input: "អស់ នឹង មាន"
• Output: ['អស់', ' ', 'នឹង', ' ', 'មាន'] ❌ (isolated spaces)

Solution Overview

Fix two issues that prevented proper space handling:

1. language.rs: Spaces were split into separate chunks → complex segmenter never saw them
2. line.rs: Complex segmenter wasn't triggered for SA×SPACE×SA sequences

Changes

components/segmenter/src/complex/language.rs

What changed: Don't split text on whitespace characters

Lines ~62 & ~105: Modified both UTF-8 and UTF-16 iterators to skip whitespace when checking for language changes

Effect: Khmer phrases with spaces stay together as one chunk: "អស់ នឹង" instead of "អស់", " ", "នឹង"

---

components/segmenter/src/line.rs

What changed: Handle SA×SPACE×SA (complex script + space + complex script) sequences

4 changes:

1. ~Line 1070: Add peek_past_spaces_for_sa() helper

   • Looks ahead past consecutive spaces to check if SA continues

2. ~Line 880: Extend complex breaking trigger

   • Changed from: only trigger for SA × SA
   • Changed to: trigger for SA × SA OR SA × SPACE × SA

3. ~Line 908: Suppress UAX#14 breaks

   • Don't break at SA × SP if SA continues after space(s)

4. ~Lines 1165 & 1198: Include spaces in text collection

   • Complex segmenter sees full phrases with spaces: "អស់ នឹង"

Effect: Complex segmenter (LSTM/dictionary) handles the entire SA×SPACE×SA sequence intelligently

---

Result

• Before: [0, 9, 10, 19, 20, ...] (double breaks)
• After: [0, 9, 19, 29, ...] (single breaks)
• Spaces properly included with words: ['អស់', ' នឹង', ' មាន'] ✅

Impact

Fixes line breaking for Khmer. Also possibly Thai, Lao, and Myanmar scripts.
Matches ICU4C behavior.

[CLAassistant]

All committers have signed the CLA.

[sffc]

Thanks for the contribution!

Please add tests for this behavior. Also, please assert that word break continues to break around spaces, and only line break gets the new behavior.

[sffc]

Please add tests.