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.

tests/lighter/signer.test.ts

@@ -50,6 +50,6 @@ describe("LighterSigner", () => {
       expect(signed.txHash.length).toBeGreaterThan(0);
     }
     expect(typeof signed.signature).toBe("string");
-    expect(signed.signature.length).toBeGreaterThan(0);
+    expect(signed.signature?.length ?? 0).toBeGreaterThan(0);
   });
 });

tests/maker-points-defense-rest.test.ts

@@ -60,10 +60,11 @@ describe("MakerPointsEngine defense-mode REST polling", () => {
         price: "100",
         origQty: "1",
         executedQty: "0",
+        stopPrice: "0",
         time: Date.now(),
         updateTime: Date.now(),
-        reduceOnly: "false",
-        closePosition: "false",
+        reduceOnly: false,
+        closePosition: false,
       },
     ];
 

tests/order-coordinator.test.ts

@@ -89,7 +89,7 @@ describe("order-coordinator", () => {
       timers,
       pending,
       "BUY",
-      100,
+      "100",
       1,
       log,
       false