feat: reuse shared panel processing across forecasters (refresh of #2)

[elmartinj]

This is a refreshed version of #2 (feat: use common syntaxis and improve speed), rebased onto current main. The original commits by Azul are preserved on this branch; the final commit fixes what was blocking the old PR.

What's in it:

• Preprocess the input DataFrame once into a shared PanelData and reuse it across model calls instead of every wrapper re-doing sorting, datetime conversion, and dataset construction.
• Thread the optional panel through the base Forecaster.forecast signature and MultiModelForecasterMixin.forecast, and update every built-in model wrapper (including Tafsut and TimesFM 3.0, which landed on main after the original PR).
• Fix the test failures from feat: use common syntaxis and improve speed #2's last CI run: quantile columns are now assigned one at a time because utilsforecast.assign_columns rejects list values for multiple columns.
• Compute the Chronos panel once so dataset construction and finetuning inputs share it.
• Remove redundant _ = panel statements in the TimesFM 1.0 / 2.5 paths.

Local verification: pre-commit (ruff, ruff-format, mypy, toml-sort) passes and the download-free test suite passes (78 passed, 6 skipped). Full model tests still need a CI run.

Related: #2

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✅ AzulGarza
❌ elmartinj

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