fix(exchanges): restore type-check safety net and repair dead balance guards

tsconfig compiled docs/ (vendored ccxt samples) and @types/react was missing,
so 269 tsc errors buried the real ones. Scoping the project and adding the
missing type packages left 42 genuine errors in src/, which exposed two bugs:

- lighter: assertSpotBalance's buy branch and createOrder's spot-sell guard
  compared the {available, wallet} object against a number, so both guards
  were dead. Introduce SpotAssetBalance with a precomputed 'effective' field
  (Extract Class) and route all three call sites through it.
- offset-maker: the below-min-sell branch logged 'skip sell' but pushed the
  SELL order anyway, and pushed a possibly-null price. Both branches now
  match their working sibling.

Also: widen LighterOrder's is_ask/reduce_only to BooleanFlag (the wire format
flags.ts already parses), extract sellableBase (Extract Function, 3 copies),
delete two scripts importing a module that does not exist, add a typecheck
script. tsc --noEmit: 269 -> 0 errors; 218 tests still pass.

package.json

@@ -16,6 +16,7 @@
     "start": "bun run index.ts",
     "lint": "oxlint",
     "lint:fix": "oxlint --fix",
+    "typecheck": "tsc --noEmit",
     "test": "bun x vitest run",
     "test:exchange-contract": "bun x vitest run tests/exchange-contract-suite.test.ts tests/exchange-factory.test.ts tests/config.test.ts",
     "test:watch": "bun x vitest",
@@ -29,6 +30,8 @@
   },
   "devDependencies": {
     "@types/bun": "^1.3.9",
+    "@types/react": "^19",
+    "@types/ws": "^8.18.1",
     "oxlint": "^1.54.0",
     "vitest": "^4.0.18"
   },