Add parity function wave 1 and regenerate reference docs

[PSU3D0]

Summary

This lands the first parity wave for Formualizer and regenerates the reference docs in one PR.

Engine/runtime additions

• statistical compatibility batch:
  • AVERAGEA, MAXA, MINA
  • STDEVA, STDEVPA, VARA, VARPA
  • SKEW.P
  • T.DIST.RT, CHISQ.DIST.RT, CHISQ.INV.RT
  • F.DIST.RT, F.INV.RT
  • BETA.INV, BINOM.DIST.RANGE, BINOM.INV
  • GAMMA, GAMMA.INV, GAMMALN, GAMMALN.PRECISE
• date/business-day additions:
  • NETWORKDAYS.INTL, WORKDAY.INTL
  • follow-up fix: existing NETWORKDAYS / WORKDAY now honor holiday arguments
• financial additions:
  • TBILLEQ, TBILLPRICE, TBILLYIELD
  • ISPMT, PDURATION
• compatibility microbatch:
  • SWITCH, ISNONTEXT
  • FACTDOUBLE, COMBINA
  • BASE, DECIMAL
  • ACOTH
  • CEILING.PRECISE, FLOOR.PRECISE, ISO.CEILING
  • SUMX2MY2, SUMX2PY2, SUMXMY2
• legacy lookup:
  • LOOKUP

Tests/docs

• adds dedicated JSON fixture coverage for the new parity functions
• adds docs-site-ready builtin doc comments/examples/FAQ metadata for all new functions
• regenerates reference pages / function metadata for docs-site

Validation

• cargo fmt --all
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p formualizer-eval
• cargo run -p xtask -- audit --functions ... for the new parity functions (41 passing, 0 failing)
• cargo run -p xtask -- schema --functions ...
• cargo run -p xtask -- ref
• cd docs-site && pnpm build

Notes

• This is intentionally one PR including both engine changes and generated docs/reference updates.
• The docs diff is broader than just the newly added pages because xtask ref --apply recalculates related links, category metadata, and generated function metadata across the reference site.