fix: correct final-bar support diagnostics

[luisleo526]

Summary

• stop hard-rejecting last_bar_index; it already lowers to the runtime final-index accessor, so warn about fed-window/chart-history divergence instead
• correct final-bar diagnostics for chart-scope barstate.islast and barstate.islastconfirmedhistory
• explicitly retain and document the narrower request.security() history-index false-fold caveat
• move the last-bar-index gate fixture from rejected to accepted-with-warning and add adversarial lowering tests

Validation

• focused: 292 passed
• full compile-enabled pytest: 1625 passed, 1 skipped (254 corpus strategies)
• gate selftest: 18 cases OK
• full parity gate: 277 fixtures OK (263 accepted, 14 rejected)
• independent review: GO after narrowing the diagnostics

Scope

This is a support-classification and diagnostic correction. It does not change generated execution semantics or the validation score. A future semantic change to the request.security() history-index folder requires separate oracle-backed work.