perf(export): stream large exports in bounded chunks instead of buffe…

[shogun444]

Closes #1179

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Why This PR Exists

Export serializers in src/lib/export-scheduler/exporter.ts previously built entire export payloads in memory as single monolithic strings before wrapping them into a Blob. Specifically:

• CSV accumulated via csvLines.join('\n')
• JSON performed JSON.stringify(data.rows, null, 2) on full datasets
• XLSX and PDF performed repeated string concatenation across large XML and HTML payloads

On large datasets, this caused heavy memory spikes and $O(n^2)$ string reallocation overhead. This PR introduces bounded chunking primitives and incremental parts assembly to eliminate monolithic string concatenation while maintaining 100% byte-for-byte output parity.

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What Changed

• Streaming & Chunking Primitives (src/lib/export/utils.ts):
  • Added ExportStreamOptions interface supporting configurable chunkSize and per-chunk progress callbacks.
  • Implemented async generators createCSVSnapshot and createJSONSnapshot to yield bounded row batches.
  • Centralized robust serialization escaping helpers: escapeCSVCell, escapeXml, and escapeHtml.
• Production Exporter Wiring (src/lib/export-scheduler/exporter.ts):
  • Refactored exportToCSV and exportToJSON to consume async generators and build the final Blob from discrete chunks.
  • Refactored exportToXLSX and exportToPDF to assemble from incremental parts arrays rather than repeated string concatenation.
  • Replaced duplicate local escaping routines with shared utilities.
• Test Coverage & Verification:
  • Added 6 unit tests in src/lib/export/utils.test.ts covering escaping, batch chunking, empty states, progress callbacks, and JSON pretty-print parity.
  • Strengthened src/lib/export-scheduler/__tests__/exporter.test.ts with a FileReader-based readBlobText helper to verify exact content equivalence rather than simple instanceof Blob checks.

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Files Changed

File	Changes
src/lib/export/utils.ts	New/Updated: Added createCSVSnapshot, createJSONSnapshot, ExportStreamOptions, and escaping helpers
src/lib/export-scheduler/exporter.ts	Wired async generators into exportToCSV/exportToJSON; converted XLSX/PDF to incremental parts arrays
src/lib/export/utils.test.ts	New: 6 unit tests for chunking, escaping, progress events, and empty-state edge cases
src/lib/export-scheduler/__tests__/exporter.test.ts	Replaced shallow Blob type checks with full byte-level content verification

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Type of Change

• Performance / Memory optimization (non-breaking change)
• Refactor / Code modernization
• Tests

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Test Evidence

Local Verification

• Formatting: prettier --check passes cleanly across all changed files
• Type Check: npm run type-check passes with 0 errors
• Linting: npm run lint passes with 0 warnings/errors
• Unit & Integration Tests: 15/15 tests passing across export test suites
• Production Build: npm run build compiled successfully (72 static pages generated)

 ✓ src/lib/export/utils.test.ts (6 tests)
   ✓ escapes CSV cells containing commas, quotes, and newlines
   ✓ yields CSV rows in bounded chunk batches with headers
   ✓ handles header-only empty CSV generation
   ✓ invokes progress callback on chunk boundaries
   ✓ produces byte-for-byte identical output to JSON.stringify(rows, null, 2)
   ✓ handles empty JSON datasets gracefully

 ✓ src/lib/export-scheduler/__tests__/exporter.test.ts (9 tests)
   ✓ exports CSV with accurate content matching snapshot
   ✓ exports formatted JSON with identical structure
   ✓ builds XLSX payload from parts array
   ✓ builds PDF payload from parts array

Test Files  2 passed (2)
     Tests  15 passed (15)
     
     

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