fix(research): keep live fetch timestamps distinct from historical cutoffs

[Pigbibi]

Summary

• Record the actual response acquisition time instead of backdating live web content to a caller cutoff.
• Return an explicit unsupported warning without fetching for historical requests; exclude unknown or future publication times.
• Keep default/current CLI research working and route explicit past end dates to the historical refusal path.

Validation

• Entire offline test suite: 64 passed, independently reproduced by the parent reviewer.
• Fixed fake clock, synthetic RSS/HTML and mocked CLI prices/output; socket and subprocess audit hooks deny external operations.
• Ruff, git diff --check, exact three-file scope and sensitive-pattern checks passed.
• Push/PR/main CI is offline; scheduled research workflows are not triggered by these events.

Boundaries

No archive was added, no historical PIT coverage or production verification is claimed, and price/theme PIT semantics remain outside this patch. No real provider/model/account/deployment calls. Unrelated artifact PR #45 remains untouched.