fix(i18n): don't treat mustache-style {{tokens}} as named substitutions

[thribhuvan003]

Problem

Since 0.2.6, messages containing mustache-style application tokens ({{targetLanguage}}) are scanned as named substitutions, so the generated types require a substitution object and a plain i18n.t("key") call fails with not assignable to parameter of type never — even when the app intentionally wants the raw localized template back (e.g. AI prompt templates rendered later by the app itself).

Fix

NAMED_SUBSTITUTION_RE now matches single-brace {name} placeholders only, using lookarounds to skip brace groups wrapped in additional braces:

/(?<!\{)\{([A-Za-z0-9_]+)\}(?!\})/g

The regex is shared by the type scanner (getNamedSubstitutionNames) and the runtime (applyNamedSubstitutions), so both stay consistent:

• {{selection}} / {{targetLanguage}} → left as literals, not scanned, not substituted
• {name} → still scanned and substituted exactly as before
• mixed messages ({greeting}, keep {{token}} as-is) → only {greeting} is a substitution

This is the first option proposed in the issue (single-brace only), which keeps 0.2.6 behavior for all existing single-brace users and restores 0.2.5 behavior for mustache templates.

Verification

Added 4 regression tests (scanner + runtime, pure-mustache and mixed cases), using the reproduction strings from the issue.

• bun run test: 49 passed (45 existing + 4 new)
• bun run check: Oxlint ✔ Publint ✔ TypeScript ✔

Fixes #2482

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