feat(engine): add units, currency, datetime, variables, and functions modules
Extracted and integrated unique feature modules from Epic 2-6 branches:
- units/: 200+ unit conversions across 14 categories, SI prefixes
(nano-tera), CSS/screen units, binary vs decimal data, custom units
- currency/: fiat (180+ currencies, cached rates, offline fallback),
crypto (63 coins, CoinGecko), symbol recognition, rate caching
- datetime/: date/time math, 150+ city timezone mappings (chrono-tz),
business day calculations, unix timestamps, relative expressions
- variables/: line references (lineN, #N, prev/ans), section
aggregators (sum/total/avg/min/max/count), subtotals, autocomplete
- functions/: trig, log, combinatorics, financial, rounding,
list operations (min/max/gcd/lcm), video timecodes
585 tests passing across workspace.
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