Add a multitask architecture module and its model sources

[LeoGrin]

Generated by Claude Code

Adds a new multitask architecture module and registers its two model versions in the loader, each backed by one checkpoint that serves both estimators. Nothing changes for the default estimator: the default model version stays v3, the README is untouched, and the new versions are opt-in through ModelVersion or a file name. The checkpoints live in a private Hugging Face repo for now, and CI reaches them through the updated HF_TOKEN.

Commits and what they do

• Architecture module, ModelVersion.V3_5 / V3_5_FAST, model sources, CPU sample limit, create_default_for_version wiring (Benjamin Jaeger, from ben/inference-update).
• create_default_for_version takes a ModelVersion in the regressor, as the classifier already does (Benjamin Jaeger).
• Model sources point at the 20260909 exports and list the multiclass variant.
• Port of fomo-fitting #3698: missing train cells are filled with the same value the cached path fills test cells with, so cached and uncached predictions agree on tables with NaNs. Regression test included; it fails without the fix.
• The layerwise-quantization test compares scales with rtol=1e-6 instead of exact equality. On macOS arm64 the two forward passes differ by 2 ulp in the absmax; the quantized keys and values are still compared exactly.
• The tabpfn_v3_5_prerelease architecture alias and its test are dropped: the released checkpoints name tabpfn_v3_5.

How this was checked

• Full non-slow suite locally on macOS with the release files cached: 2177 passed, 160 skipped, 8 xfailed.
• Each release file fit and predicted with both estimators on a table with string and datetime columns; file name resolution gives V3_5 / V3_5_FAST.
• CI dispatched on this branch: https://github.com/PriorLabs/TabPFN/actions/runs/34398005256. The model download succeeded on all jobs. Ten jobs green, including tabpfn-extensions, tabpfn-time-series and both GPU jobs. The macOS 15 job hit its 30-minute cap in "all tests" mode; on main that job already takes 28 to 29.5 minutes.

Not in this PR: making the new version the default, the README, and the docs. Those follow in a separate PR.

🤖 Generated with Claude Code

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[priorphil]

I fixed Codex's comment and restored the simpler version of the "base" name check since the global was not used anywhere else.

[jmkuebler]

stamp

[github-actions]

Reapproving after a push, as this is a PR from an internal author and was already approved.