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.

tsconfig.json

@@ -25,5 +25,10 @@
     "noUnusedLocals": false,
     "noUnusedParameters": false,
     "noPropertyAccessFromIndexSignature": false
-  }
+  },
+
+  // docs/ holds vendored third-party samples (ccxt examples); they are reference
+  // material, not compilation units, and their errors mask real ones in src/.
+  "include": ["index.ts", "src/**/*", "tests/**/*", "scripts/**/*"],
+  "exclude": ["node_modules", "docs"]
 }