Record public 0.3.1 release evidence - #40

What changed

• records all six public 0.3.1 package hashes and byte sizes
• binds the release evidence to merged commit 51b4c05
• marks the 12-asset GitHub prerelease and build provenance as passed
• links the public SHA-256 ledger directly beside the README download

Validation

• all six package hashes match the release-hosted platform checksum files
• the recommended Windows installer returns HTTP 200 with the expected byte length
• git diff --check