Preserve external stylesheet styles on native list-item leaves

[chubes4]

Closes #755

Root Cause

Normal <li> leaves were excluded from HighValueStyleBoundaryPolicy, so resolved external stylesheet declarations were not projected even though the output remained native core/list and core/list-item blocks. RichText descendants within wrapped list items also lost the source-tag marker needed for projected descendant selectors to match after serialization.

Generic Fix

Treat native list items as style boundaries, preserve wrapped nested-list topology, and materialize source-tag markers on retained list-item RichText descendants. This retains external typography, spacing, positioning, pseudo-element, and direct-descendant selector behavior without fixture-specific selectors or non-native fallbacks.

Tests

• composer run test:unit
• composer run test:canonical

Fixture 89 Final Combined Evidence

At exact SHA 50309edbc004370de3f3c2468f013b0eee4fa1ce: visual 0/10,325,760; 170/170 native; 340/340 editor; zero core/html/warnings/findings; viewport 1280x8067.

AI assistance: gpt-5.6-sol using OpenCode inspected ancestry, replayed the isolated #755 patches, ran tests, and drafted this PR. Chris Huber is responsible for every line.

[chubes4]

Updated this branch with merge commit 238c6dc from origin/trunk at fde69af.

Conflict resolution: retained both stylesheet projection contracts in artifact-author-stylesheet-projection.php: #769 native-table selector/specificity, normalization, border, and runtime-validity coverage; and #755 native list-item topology, retained RichText/source-marker styles, pseudo-elements, and Gutenberg-validity coverage.

Tests:

• composer test (from php-transformer)
• php tests/unit/artifact-author-stylesheet-projection.php
• git diff --check origin/trunk...HEAD

AI assistance: gpt-5.6-sol via OpenCode was used to diagnose, implement, test, and verify this change. Chris Huber remains responsible for every line.