fix(core): dark-surface colors for links, h6, code and mark (#23)

[vietnguyentuan2019]

Fixes #23.

Problem

In 1.11.0 only the default text color followed a dark Theme. The built-in link, <h6> and <pre><code> colors were still light-surface constants, measuring 4.1 / 3.9 / 2.5 : 1 against #121212 (<pre><code> fails even the 3:1 large-text bar), and the inline <code> / <mark> chips were bright blocks on a dark page.

Change

StyleResolver.darkSurface (default false) swaps those UA colors for dark-surface variants, applied right after the UA style so author CSS and inline styles still win. An <a> without href is untouched.

Element	Dark variant	Contrast (measured)
link	#64B5F6	8.5 on #121212, 7.5 on #1E1E1E
<h6>	#9CA3AF	7.4 / 6.6
<pre><code>	#79C0FF	9.6 / 8.6
inline <code>	#79C0FF on #262C36	7.2 on its chip
<mark>	#FDE68A on #5C4A00	6.9 on its chip

HyperViewer turns it on when the effective default text color is light, not when the theme is dark. That choice matters: the migration guide tells people to use textColor: Colors.black87 for a white pane inside a dark app, and a theme-based switch would put light blue links on white. Light Theme + light textColor (a dark pane) gets the dark palette. Light output is unchanged.

Not touched: <blockquote>, <kbd>, table headers keep their light backgrounds (their text stays readable via the 1.11.0 contrast guard; they remain light blocks on a dark page). Documented in the README and CHANGELOG; say if you want that as a follow-up.

Verification

• 11 new tests in test/dark_ua_colors_test.dart: per-element contrast on page and card surfaces; the exact light palette; the white-pane and dark-pane cases; theme toggle; author CSS / inline precedence; the compute() isolate path; markdown; one pixel test.
• Mutation-checked, four ways: never applying the variants, choosing by theme instead of text color, ignoring text luminance, and giving href-less anchors a link color each fail tests.
• Full root+core 2513, epub 53, example 44 pass; analyze and format clean; the new file also passes on Chrome. No regex lookbehind added.
• Goldens can only be verified on CI (they fail identically on clean main locally). Light-mode pixels must be unchanged.
• Released packages are unaffected; this is Unreleased in both CHANGELOGs, no version bump.

[github-actions]

⚠️ Layout Regression — Budget exceeded (informational)

	Fixture	Budget (ms)	Median (ms)	P95 (ms)
❌	simple_paragraph	8	16	29
❌	mixed_inline	10	14	14
❌	float_layout	12	14	27
❌	table_20_rows	14	42	61
✅	cjk_ruby	14	9	11
❌	large_article	16	33	63

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.