fix: release pipeline must always publish version greater than already published

[konard]

Summary

Fixes #22 — The auto-release pipeline was silently accepting "version already exists" on crates.io as success, when it should always publish a version strictly greater than what's already published.

Root Cause

Three interacting bugs in the CI/CD release scripts:

1. publish-crate.rs treated "already exists" as success (exit 0), masking the real failure
2. version-and-commit.rs did not check crates.io for already-published versions — when a git tag existed, it stopped bumping instead of finding the next unpublished version
3. check-release-needed.rs did not output the max published version from crates.io for diagnostic/coordination purposes

Changes

• publish-crate.rs: "already exists" is now a hard failure (exit 1) with a clear error message explaining the pipeline must compute a new version
• version-and-commit.rs: Now queries crates.io for the maximum published version and ensures the bumped version strictly exceeds it. If the initial bump produces a version ≤ published, it adjusts to max_published + 0.0.1. Also checks git tags and crates.io in a loop as a safety net
• check-release-needed.rs: Now outputs max_published_version for downstream diagnostic use
• Case study: Full timeline reconstruction and root cause analysis in docs/case-studies/issue-22/
• Experiment script: Verifies version bumping logic against various published version scenarios

How It Prevents Recurrence

The fix operates at two levels:

1. Prevention: version-and-commit.rs guarantees the new version exceeds all published versions before tagging
2. Safety net: publish-crate.rs fails loudly if a version collision somehow still occurs, preventing silent false-positive releases

Test plan

• All existing tests pass (cargo test --all-features)
• cargo fmt and cargo clippy clean
• File size check passes
• Experiment script validates all version bumping edge cases
• Manual verification: check-release-needed.rs correctly reports max published version (0.2.0)
• Manual verification: version-and-commit.rs --bump-type minor correctly produces 0.3.0 (> 0.2.0)
• Manual verification: version-and-commit.rs --bump-type patch correctly produces 0.2.1 (> 0.2.0)
• CI pipeline passes on this branch

🤖 Generated with Claude Code

[konard]

🤖 Solution Draft Log

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• Additional models:
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[konard]

✅ Ready to merge

This pull request is now ready to be merged:

• All CI checks have passed
• No merge conflicts
• No pending changes

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