Prepare v0.3.0 contributor release

[kwisatzh]

Changes

• Finalize release notes and the v0.3.0 installer default.
• Check user/session answers from every native packaged binary, including missing usage, older windows, strict share thresholds and hidden hashes.
• Document compatibility and immutable release verification.

Verification

• Go 1.26.8 race tests and vet passed.
• Four development archives built; checksums and native macOS ARM64 smoke/top-k checks passed.
• Real collector export to the packaged fleetdiff binary passed.
• Hosted native-platform and security checks are required before merge.