Add full inventory reconciliation coverage

[Bogyie]

Summary

• add a machine-checkable reconciliation test that accounts for all 338 bundled official KIS inventory endpoints exactly once
• document the final reconciliation surfaces and clarify the distinction between narrow typed methods and inventory-backed SDK-callable APIs
• update the English README with the completed inventory-backed SDK surface, coverage table, validation commands, and Korean usage guide link
• add docs/usage-ko.md as a separate Korean usage guide covering practical SDK usage, live trading safety boundaries, and endpoint coverage status

Verification

• cargo fmt --check
• cargo test --locked full_inventory_reconciliation_accounts_for_every_official_endpoint_once
• cargo test --locked
• cargo test --locked --test mock_server_contract
• cargo doc --locked --no-deps
• git diff --check
• documentation QA confirmed README.md, docs/usage-ko.md, referenced docs links, placeholder-only credentials, live-trading guard wording, and 338/338 coverage limitations

Notes

• Reconciliation is based on the bundled captured inventory from BOG-221; this PR does not perform a live portal re-scrape.
• Not every endpoint has a narrow typed request/response struct. The acceptance boundary here is SDK-callable coverage through typed methods, scoped inventory APIs, or lower-level execute_inventory.
• This change updates tests and documentation only. It does not add live credentials, production account data, live API calls, or live order execution.

[amazon-q-developer]

This PR adds comprehensive inventory reconciliation coverage that successfully validates all 338 endpoints are properly accounted for across SDK surfaces. The implementation is solid with appropriate test coverage and clear documentation updates. No blocking issues identified - the code is ready to merge.

---

You can now have the agent implement changes and create commits directly on your pull request's source branch. Simply comment with /q followed by your request in natural language to ask the agent to make changes.