Prepare v0.1.2 release

[ian-pascoe]

Summary

• Bump the crate, embedded catalog, and version assertions to 0.1.2; link release notes.
• Document the fixes since v0.1.1 and stricter legacy Preview/Recovery and external-Document checks.
• Add the required immutable v0.1.1 compatibility seed, extend existing checks to both earlier releases, and test tamper rejection. Original v0.1.0 fixtures remain unchanged.

Validation

• 155 Rust tests passed; formatting and Clippy passed.
• Schema, stored-state, and complete release-gate checks passed.
• 11 release-script tests passed; Windows-only PowerShell check skipped locally.
• Both stored-state deserialization tests passed against the immutable v0.1.1 source tag before copying its format-equivalent fixtures.

After CI passes, merge and tag v0.1.2 to run the full release matrix and publish through the existing gated workflow.