Allow publication bounds in price history exports

[kvkenyon]

The DAM/RT history-export example previously downloaded all retained sources even when users requested a narrow delivery-date range. Add optional --posted-from and --posted-to bounds so users can explicitly limit publication selection and downloads.

The default still retrieves maximum retained history. Delivery-date filters remain independent and apply after decoding. Documentation explains that publication bounds exclude bundle-only reports without original archive metadata and corrections published outside the chosen range. Invalid publication bounds fail before overwriting the output.

Validation:

• 35 backfill tests pass, including bounded DAM/RT exports that skip unrelated monthly bundles and preserve the selected archive.
• Strict mypy passes for 43 source files; Ruff and CLI help pass.
• An authenticated bounded export retrieves 24 Houston DAM hours for delivery on May 2, 2014, using publications from May 1–2. This is a workflow check, not a new complete-history comparison.

No runtime client or dependency changes. Follow-up to merged #8.