data grid: dead code, perf, security, closure capture fixes

Phase 1: extract magic numbers to constants, remove dead code
Phase 2: replace O(n*m) column scans with map sets, eliminate
duplicate presentation_rows computation in keydown closure
Phase 3: reject NaN/Inf in XLSX export, rune-aware PDF clipping
Phase 4: reduce cfg-capturing closures from ~24 to 3

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mike-ward