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.

calcpad-engine/Cargo.toml

@@ -7,7 +7,12 @@ edition = "2021"
 crate-type = ["cdylib", "staticlib", "rlib"]
 
 [dependencies]
-chrono = "0.4"
+chrono = { version = "0.4", features = ["serde"] }
+chrono-tz = "0.10"
 dashu = "0.4"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+ureq = { version = "2", features = ["json"] }
+
+[dev-dependencies]
+tempfile = "3"