fix: singularize/pluralize regular -s nouns like gas/lens (fixes #123)

[youdie006]

Fixes #123.

Root cause

singularize() applies its rules first-match, front-to-back. For the small class of regular nouns whose singular already ends in -s and whose plural adds -es (gas/gases, lens/lenses), no -es rule matches, so the word falls through to the generic tail rule /s$/ -> '' and gets over-stripped:

inflection.singularize('gases');  // 'gase'  (expected 'gas')
inflection.singularize('lenses'); // 'lense' (expected 'lens')

The same generic rule also mangles the already-singular form (gas -> ga), so the singularize(pluralize(x)) === x round-trip is broken as well.

Fix

Adds start-anchored, bidirectional rules for the safe -s -> -ses class gas|lens|bias|atlas|iris, following the existing summonses/nucleuses convention already in the file (anchored plural convert rule + singular protect rule, symmetric in both directions). Anchoring ^...$ is required so the bare protect rule does not wrongly protect words that merely end in the stem (sagas/omegas/cameras), and so houses -> house / buses -> bus keep working.

This is the same category as the recently merged #125 (analyses -> analysis). If a narrower change is preferred, the class list trims cleanly to just gas|lens.

Verification

• Red/green: with the fix reverted, the new gases/lenses assertions fail (gase/lense); with the fix, all tests pass.
• New tests cover both directions, the round-trip, the must-not-regress cases (houses, buses), and the must-not-over-protect cases (sagas, omegas, cameras, analyses).
• npx vitest run -> 15 passed; npx tsc --noEmit -> clean.

Note: this contribution was prepared with AI assistance and reviewed by me before submission.