Add TypeScript strict mode and enable all strict checks

[big14way]

Ensure the SDK compiles cleanly with all TypeScript strict checks enabled.

Acceptance Criteria:

• Enable all strict options in tsconfig: strictNullChecks, strictFunctionTypes, etc.
• Fix any resulting type errors
• Add noUncheckedIndexedAccess
• Verify all exports maintain correct types

Files: tsconfig.json, src/*.ts