feat(finance): XLSX + CSV + Tally exports (#99 pt.1)

[ankitsejwal]

M3.5 · Task #99 (part 1) — XLSX / CSV / Tally exports

The most-demanded treasurer feature — financial reports a CA can consume. Dependency-free and Cloudflare-Workers-safe (no lockfile change).

Worker-safe XLSX writer (lib/xlsx.ts)

exceljs/SheetJS assume Node APIs, so I wrote a small from-scratch .xlsx writer: real OOXML SpreadsheetML zipped with STORED (uncompressed) entries + a hand-rolled CRC32 — no zip/deflate dependency. Validated locally with unzip -t (CRC OK, no errors) and structurally (all 6 OOXML parts).

Export layer (shared/export.ts, pure + tested)

• report → table (rows, in rupees) for Trial Balance, I&E, Balance Sheet, R&P, and dues
• CSV serializer (RFC-escaping)
• Tally-import XML builder: ledger masters + vouchers, debit negative / credit positive (Tally convention)

API

• ?format=xlsx|csv on /reports/{trial-balance,income-expenditure,balance-sheet,receipts-payments} and /bills/dues
• GET /reports/tally-export → Tally XML (PAYMENT→Receipt, EXPENSE→Payment, else Journal), importable into TallyPrime

Acceptance

• ✅ Every financial report downloads as .xlsx (CRC-verified valid zip / OOXML) — recommend a real-Excel smoke test on deploy; pip/openpyxl is locked down in this sandbox.
• ✅ Tally XML for receipt + payment + journal vouchers.
• ⏳ Print-ready statement PDFs + the invoice ₹-glyph fix → next export PR (feat(finance): penalties & interest on arrears (#95) #99 pt.2).

Tests

xlsx zip structure + inline-string/number/escaping; CSV escaping; Tally envelope/signing/masters; table shaping. Suite 359 green. No schema change.