PT-4313: Mirror the core review of the gutter coverage test

[irahopkinson]

Summary

Follow-up to #10 (PT-4313). The review of paranext-core's twin PR, paranext/paranext-core#2827, found things in the derived gutter coverage test that also apply here, plus two stale focus-box comments in the stylesheet. This carries the same changes so the twins stay in step. Test and comments only; no rendering change.

Test (usj-nodes.css.gutter-coverage.test.ts)

• The parser-sanity floors are back (> 40 and > 20 entries). Both sides of the derived comparison come from the same parser, so a parser that started reading less would shrink them in step; > 0 only catches reading nothing.
• The nesting guard walks only rules that name a marker class, and fails a tracked property set anywhere but directly in a top-level marker rule (inside an at-rule, or an at-rule nested in the rule). An unrelated @media print { .note { margin: 0 } } no longer trips it.
• [dir=…] is read in any quoting or case, with or without a case flag, plus :dir(); a direction inside :not() is read as the other direction.
• directionAsymmetries reports a value set in only one direction, for margins and for text-indent.
• No hanging indent may reach past its marker's margin. That is what lets the focus-box comments say the marker's own column wins the min() for every marker in this file, so the claim is pinned rather than left to go stale.
• Each usfm.sty oracle is compared as one object; the gutter maps are compared with toEqual.
• The header carries a "Known limits" paragraph naming what the derivation does not model (class-token classification rather than DOM resolution, blanket .para rules, the cascade approximated as direction-qualified-beats-agnostic, padding untracked, calc() unreadable, direction from [dir=…]/:dir() only, agnostic margin-right counted as inline-start).

Stylesheet (usj-nodes.css)

• The two focus-box comments say what --verse-text-start does today: a fallback start that the marker's own column beats for every marker in this file, rather than "aligning the box with the poetry text start".

Verification

• usj-nodes.css.gutter-coverage.test.ts 4/4 and usj-nodes.css.test.ts 9/9.
• Mutation probes against usj-nodes.css behave as intended: RTL-only margin, at-rule wrapping a marker rule, at-rule nested inside a marker rule, direction-qualified gutter rule, calc() value, hanging indent without margin, real margin in a compensation rule, agnostic margin-right, !important, :not([dir='rtl']) and a hanging indent past its margin all fail; an unrelated nested rule, a hanging indent within its margin and the unchanged sheet pass.
• ESLint and Prettier clean on the changed files.

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