hotfix: bump petsc 3.21.5 → 3.24; add v3.1.0 Binder badge to README

[jcgraciosa]

• environment.yaml: bump petsc/petsc4py 3.21.5 → 3.24 to fix the command-line container build (DMCopyFields signature mismatch). Same fix already on development via PR fix(container): bump environment.yaml dependencies to match pixi.toml #356; pixi.lock already uses 3.24.
• README.md: add v3.1.0 Binder badge; mark v3.0.0 as previous release.

Build tested on fork via workflow_dispatch (run 30358340515) — compilation succeeded, only push failed due to fork permissions.

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Underworld development team with AI support from Claude Code

[lmoresi]

Adversarial review

We verified the mechanics end-to-end. Findings, none blocking:

• environment.yaml: partial cherry-pick of #356 — bumps petsc/petsc4py only, leaves python <= 3.11, numpy<2, cython 3.*, while development carries python=3.12.*, numpy>=2,<2.5. Works as-is (fork run 30358340515: env solved on python 3.11, Successfully installed underworld3-0.0.0), but guarantees a trivial environment.yaml conflict at the next release merge.
• The badge pulls branch=main of underworld3 into the frozen v3.1.0 launcher image, so notebooks can drift ahead of the installed 3.1.0 — same pattern as the existing v3.0.0 badge, inherited not new.
• Note: the petsc bump doesn't affect Binder at all (launcher uses the prebuilt GHCR base image); it fixes the command-line container built from main, as the PR body says.

Attacks that failed: the v3.1.0 launcher ref exists (branch, alongside v3.0.0/v3.0.1); the fork build fully succeeded — the only failure was the expected permission_denied pushing to GHCR from a fork; amd64-only fork test is representative since main's workflow builds amd64 only.

Clean to land as a main hotfix (release-branch merge, so we're leaving the button to Louis).

[lmoresi]

@jcgraciosa cleared to merge — review above found nothing blocking (fork build verified end-to-end, launcher ref exists). Go ahead and land it when you're ready. Only heads-up for later: the environment.yaml delta will conflict trivially with development at the next release merge.