chore: bump toolchain to v4.34.1

[kim-em]

This PR moves the toolchain from v4.34.0 to v4.34.1, picking up that release's runtime fixes, and repins mathlib and doc-gen4 at their v4.34.1 tags (both are tagged for this patch).

No source changes were needed. lake build is clean locally, 9589 jobs.

🤖 Prepared with Claude Code

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[jstoobysmith]

Approved. Will merge now.

[jstoobysmith]

I updated the ReadMe accordingly. Will merge once the linters pass again.