fix(inflekt): singularize coined -is/-us plurals (apis -> api)

[pyramation]

Summary

singularize treated every -is/-us word as already singular, so a coined plural of a vowel-final stem never lost its s and pluralizing it again doubled up — apis → apises, which is how it surfaced (a getGraphQLQueryName('apis') in constructive returned apises).

-const ALREADY_SINGULAR_REGEX = /(?:is|us)$/i;   // "no English plural ends in is/us"
+const ADJECTIVE_OUS_REGEX = /ous$/i;            // anxious, various — no noun stem ends in "ou"

The premise was wrong, and no narrower regex fixes it: apis and iris have the same shape (a vowel plus s), as do menus and radius. Which one a word is, is a dictionary fact, so the real singulars now come from SINGULAR_EXCEPTIONS as self-mappings (status: 'status', iris: 'iris', radius: 'radius', 166 of them, all dictionary-proven by scripts/generate-exceptions.ts), and words no dictionary knows fall through to the generic "drop the s" — which is what identifiers want: apis → api, cpus → cpu, uris → uri.

Those new self-mappings brought their plurals under the table's existing round-trip test and exposed a gap it had never covered (aegis → aegises → aegise, since -ses is only -sis for the Greek nouns), so the generator gained a closure pass for singulars that themselves end in s. Net table change: 747 → 743 entries, with the 212 removed entries (alibis: 'alibi', antis: 'anti', …) now handled by the generic rule instead of being listed.

pluralize is untouched and byte-identical to 0.7.2 on every word checked; only singularize changes, and only for -is/-us.

Testing

• pnpm test in packages/inflekt: 107 passed, including the new -is/-us cases (coined plurals, dictionary singulars left alone, dictionary vowel-stem plurals still depluralized).
• scripts/generate-exceptions.ts is idempotent: rerunning reproduces the committed table.
• pnpm build and pnpm lint clean across the monorepo; pnpm -r test shows no failures caused by this (schema-ts-cli/yamlize-cli fail on "no tests found", which is unrelated and pre-existing).

Link to Devin session: https://app.devin.ai/sessions/c2c27132839a45219a440fd10cbf13a6
Requested by: @pyramation

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