1.13.1: slimmer archive and documentation corrections

[vietnguyentuan2019]

Packaging and docs only; no library code changes.

• .pubignore: drop packages/ and the unused recording entry point from the root archive (32 MB → 18 MB).
• Every widgetBuilder: (context, node) example did not compile (the callback is (UDTNode node)); node.isImage does not exist. Fixed in README, MIGRATION, performance and security guides, and the formula demo snippet (also HyperViewer.delta).
• box-sizing and position: relative were documented as supported but are not applied (measured with a layout test). Matrix, SUPPORTED_HTML and LIMITATIONS corrected; both rows pinned in docs_matrix_sync_test.
• Version 1.13.1 (root only; core stays 1.13.0). 2610 root+core tests pass.

[github-actions]

⚠️ Layout Regression — Budget exceeded (informational)

	Fixture	Budget (ms)	Median (ms)	P95 (ms)
❌	simple_paragraph	8	18	25
❌	mixed_inline	10	16	18
❌	float_layout	12	19	24
❌	table_20_rows	14	43	60
✅	cjk_ruby	14	10	12
❌	large_article	16	55	83

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.