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.

src/exchanges/paradex/gateway.ts

@@ -579,7 +579,7 @@ export class ParadexGateway {
       const isClosePosition = (extraParams as any).closePosition === true;
       if (isClosePosition) {
         const posAbs = this.getCurrentPositionAbs();
-        if (Number.isFinite(posAbs) && posAbs > 0) {
+        if (posAbs != null && Number.isFinite(posAbs) && posAbs > 0) {
           amount = posAbs;
         }
         const current = Number(amount);