1.13.0 pre-release review: candidate-bound fix, stress/fuzz tests, docs

[vietnguyentuan2019]

Final review pass before publishing 1.13.0. Nothing is published or tagged.

Fix

_safeCandidateCharLimit (from #29) guessed the candidate-prefix bound from whether the first 32 characters contained CJK, and ignored the text scaler. A Latin run after one CJK character (<b>注</b>日本語 english …) was cut at ~83 characters, leaving the line up to 18 glyphs short. Before #29 the same input filled the line. The bound now comes from walking the actual text with each character's narrowest width. Reproduced with TextScaler.linear(0.5) at 800 px, because the test font (Ahem) makes every glyph 1 em wide and hides this class of bug.

Tests (+49)

• line_breaking_stress_test.dart: narrow glyphs, no character lost or duplicated, 0 / 1 px widths, padding and floats wider than the container, intrinsic sizes in table cells and inline-blocks, 200,000-character paragraphs, a 100,000-character paragraph on the bounded fallback path.
• fuzz/layout_fuzz_test.dart: lays mutated documents out at 0, 1, 37 and 300 px (the existing fuzz suite only parsed).
• char_range_and_hooks_test.dart: ruby ranges, the ellipsis-only line, TextPainter layout hooks as a clock-free per-line bound.
• Each was mutation-checked. Also passing on Chrome.
• Coverage of lines changed since v1.12.0: 94.4% (322/341). The remaining lines are defensive branches that cannot be reached with the current fragment types.

Docs

• CSS matrix claimed word-break: keep-all as supported; nothing implements it. Corrected, and overflow-wrap added.
• ROADMAP: 1.13.0 marked "Release candidate" (flip to Shipped when published).
• New doc/TESTING.md; test/integration/README.md listed six files that do not exist.

Checked, no change needed

• All 7 other packages: lib/ is byte-identical to what is on pub.dev and their tests pass against core 1.13.0. Only core and root need a release.
• dart pub publish --dry-run: core 0 warnings. pana on core: 160/160. Root pub downgrade + analyze clean.
• Main CI on ef062d9: Ubuntu stable/beta, Windows, macOS all green.
• Demo app: 44 widget tests; integration test opens 27 demo screens with 0 errors; RTL, CJK, mixed, float, break-all and ellipsis scenes rendered on macOS with real fonts and inspected.

[github-actions]

⚠️ Layout Regression — Budget exceeded (informational)

	Fixture	Budget (ms)	Median (ms)	P95 (ms)
❌	simple_paragraph	8	17	28
❌	mixed_inline	10	15	18
❌	float_layout	12	18	21
❌	table_20_rows	14	46	68
✅	cjk_ruby	14	10	12
❌	large_article	16	51	69

One or more fixtures exceeded the 16 ms budget.

Flutter 3.41.5 · ubuntu-22.04

Informational, does not block merge: CI runners use software
rendering and are 2-3x slower than the devices the budgets are
calibrated for. Compare against the base branch; if a fixture
regressed relative to it, profile with:

flutter test benchmark/layout_regression.dart --reporter expanded

and check _performLineLayout / _buildCharacterMapping for
any new O(N²) or O(N log N) paths introduced in this PR.