Fix secondary highlight handling for partial selections

[k-undurkhaan-2]

Summary

• Require an exact selection of one primary highlight before promoting it to the secondary color
• Treat partial, mixed, multiple-highlight, and already-secondary selections as deterministic no-ops while clearing stale toolbar state
• Preserve adjacent and nested-inline primary/secondary ranges during snapshot restore
• Add Chromium regression coverage for selection boundaries and persistence

Root cause

updateSelectionToolbar() treated any Range.intersectsNode() result as a full highlight match, so applySelectionHighlight() recolored the entire span and discarded the saved range. Boundary-only contact also counted as overlap. Highlight restore additionally assigned seam offsets to the preceding text node and did not expand fully selected inline boundaries, causing adjacent or inline ranges to be lost.

Impact

Partial or cross-boundary selections no longer recolor unselected text or create nested highlights. Complete primary-highlight selections still promote to secondary, and resulting highlight levels survive snapshot/restore.

Validation

• node developer/tests/js/readingInteractionRegression.test.js
• npm test --prefix developer (72 tests)
• python -m unittest discover -s developer/tests/py -p "test_*.py" (20 tests)
• python developer/tests/ci/check_reading_data_integrity.py
• python developer/tests/e2e/e2e_runner.py
• node scripts/build-bundles.mjs --check
• git diff --check

Fixes #115

[githubSINGLE]

all things look good to me