Data Loader Polish and Strategy Pipeline Docs

[Muratich]

Deferred from old #29/#30: no full validation-dataset split or incremental sync in sprint.

Scope:

• Close remaining gaps in validate_candles() (document what is / isn't checked)
• End-to-end doc: broker → loader → price series → engine → metrics
• Fix any bugs found during optimizer integration testing
• load_price_series(..., last_n_bars=)

Definition of Done:

• src/data_loader/README.md describes composable pipeline path
• No regression in existing loader tests
• Incremental updates / validation datasets explicitly marked post-course