fix(core): block plugins on custom tags were laid out at 0x0 and never painted

[vietnguyentuan2019]

Problem

Found by running the demo app for the 1.11.0 release check: the Plugin API demo logged Layout Warning: More child widgets than fragments. Node: info-box and (on inspection) <info-box> / <rating> rendered as nothing.

A custom tag (<info-box>, <x-badge>) has no UA display style, so the HTML adapters build it as an InlineNode. _handleInlineNode only consulted the inline plugin tags, so a registered block plugin on such a tag never reached _tokenizeBlockPlugin: the widget was built, linked to no fragment, laid out at 0x0 and never painted. Only tags that were already block (figure, div) worked — which is what the old tests used.

Reproduced on clean main (probe: SizedBox(height: 40, ColoredBox(amber)) as a block plugin on <x-a> → size 0x0, 0 amber pixels).

Fix

A registered block tag is handled as a block whatever its node type (_handleInlineNode checks _blockPluginTags first). 9 lines in render_hyper_box_layout.dart.

Why the old tests missed it

They hand-built BlockNode(tagName: 'figure'), or asserted findsOneWidget, which a 0x0 widget satisfies. The new tests (test/block_plugin_custom_tag_test.dart) assert size, position between the neighbouring paragraphs, painted pixels, stacking, virtualized mode, that the plugin owns its children, and that unregistered and inline plugins are unaffected.

Verification

• 7 new tests; mutation-checked (removing the fix fails 4 of them).
• Full suite on this branch: 2451 pass. Analyze and format clean.

Not a regression: present on main / 1.10.0. Candidate for the next release.

[github-actions]

⚠️ Layout Regression — Budget exceeded (informational)

	Fixture	Budget (ms)	Median (ms)	P95 (ms)
❌	simple_paragraph	8	17	29
❌	mixed_inline	10	15	18
❌	float_layout	12	16	25
❌	table_20_rows	14	46	61
✅	cjk_ruby	14	10	12
❌	large_article	16	40	64

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.