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.

.claude/settings.local.json

@@ -4,7 +4,26 @@
       "Bash(find:*)",
       "Bash(ls:*)",
       "Bash(./run-pipeline.sh --phase1 --dry-run 2>&1 | sed 's/\\\\x1b\\\\[[0-9;]*m//g')",
-      "Bash(git branch:*)"
+      "Bash(git branch:*)",
+      "Bash(kill 5354 5355 5360)",
+      "Bash(git worktree:*)",
+      "Bash(while read:*)",
+      "Bash(do git:*)",
+      "Bash(done)",
+      "Bash(git ls-tree:*)",
+      "Bash(cargo build:*)",
+      "Bash(source ~/.zshrc)",
+      "Bash(source \"$HOME/.cargo/env\")",
+      "Bash(export PATH=\"$HOME/.cargo/bin:$PATH\")",
+      "Bash($HOME/.cargo/bin/cargo build:*)",
+      "Bash($HOME/.cargo/bin/cargo test:*)",
+      "Bash(git status:*)",
+      "Bash(git add:*)",
+      "Bash(git rm:*)",
+      "Bash(git commit:*)",
+      "Bash(/Users/cassel/.cargo/bin/cargo test:*)",
+      "Bash(tee /tmp/test-output.txt)",
+      "Bash(echo \"EXIT: $?\")"
     ]
   }
 }