feat(nullpii): mask template syntax ranges from PII detection

[lBroth]

Summary

After #43 stripped the PUA sentinels around user-authored {{...}}, the GLiNER model still tagged template variable names (short-kebab-case-slug, user_name, …) as private_person. Vault substitution then nested the PII placeholder inside the user's template, producing bracket-broken output like {{{{PII_PRIVATE_PERSON_*}}}}.

This adds a post-filter that finds the syntactic ranges of common templating dialects and drops any span overlapping one of them:

• {{ ... }} — Mustache / Handlebars / Vue / Jinja2 expression
• ${ ... } — JS / TS template literal
• <% ... %> — ERB / EJS
• {% ... %} — Jinja2 / Twig statement

Partial overlap counts — a span crossing }} would also corrupt bracket count after vault substitution.

Files

• src/template-mask.ts (new) — findTemplateRanges, dropSpansInsideTemplates
• src/nullpii.ts — wire post-applyThresholds, pre-refineSpanBoundaries
• test/template-mask.test.ts (new) — 11 unit tests
• test/nullpii.test.ts — integration: recognizer FP inside {{...}} gets masked

Test plan

• npm test — 283 passing (was 271)
• npm run typecheck / lint / build — clean
• Repro user's gateway report case: system prompt with {{short-kebab-case-slug}} survives round-trip without nested-brace garbage

Notes

• Template ranges computed on the original input — escape is length-preserving so offsets transfer to the escaped coord space spans live in.
• Non-greedy regex match — nested {{ {{x}} }} resolves on inner pair (acceptable trade-off; outer becomes bare braces which GLiNER no longer sentinel-confuses).

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