[codex] Use tariff-specific MPSC PSCR factors

[javaDevJT]

Summary

Adds Rider 18 export-rate support matching DTE's Rider 18 spreadsheet math, with tariff-specific PSCR factors sourced dynamically from the MPSC DTE electric rate-book PDF.

• Calculates non-net-metering export credits as parsed generation components plus the selected tariff's MPSC PSCR factor.
• Parses PSCR factors for all tariff rows in C8.5 of the MPSC DTE electric rate book: https://www.michigan.gov/-/media/Project/Websites/mpsc/consumer/rate-books/electric/dte/dtee1cur.pdf.
• Preserves net-metering behavior by using the full import-equivalent rate.
• Exposes selected PSCR, selected PSCR rate code, PSCR source URL, and the parsed pscr_rates map on export entity attributes.
• Adds calculator, MPSC PSCR PDF parser, config-flow, and sensor coverage plus in-repo research notes.

Closes #1.

D1.11 Validation

Using live DTE PDF generation components and PSCR 1.877¢/kWh from the MPSC C8.5 summary row for D1.11 Standard TOU, calculated export credits match the Rider 18 workbook values exactly:

Period	Calculated	Workbook
Summer Off-Peak	10.586¢/kWh	10.586¢/kWh
Summer On-Peak	16.284¢/kWh	16.284¢/kWh
Winter Off-Peak	10.586¢/kWh	10.586¢/kWh
Winter On-Peak	12.196¢/kWh	12.196¢/kWh

Validation

• pytest -q -> 33 passed
• python -m compileall -q custom_components tests
• git diff --check

Notes

The repository does not have an origin/main branch. The feature branch was rebased onto the freshly fetched default branch, origin/master, and this PR targets master.