chore: harden cross-platform audit evidence

[sanskarIN]

Summary

Continues post-source-completion maintenance without reopening the Version 2.0.12 feature scope.

• version the cross-platform audit JSON contract and keep a stable six-target result shape;
• reject ambiguous empty --root= input;
• fail closed if Android, Linux, Windows, macOS, or unsigned-iOS qualification packages lose required SHA-256 sidecars;
• retain successful machine-readable release/readiness/repository/platform/source-completion reports in CI with pipe-failure propagation;
• add process/regression coverage for audit schema, invalid roots, native checksums, CI evidence retention, and Phase 34 continuity;
• synchronize cross-platform support docs, maintainer tooling docs, changelog, and what_changed.md.

Release boundary

Version remains 2.0.12+2012. The canonical Phase 32 source-completion contract remains protected. Stable real-world qualification remains 0/13; this PR does not invent physical-device, assistive-technology, PWA lifecycle, signing/provisioning, native-branding, external-handler, or store evidence.

Verification

This execution environment does not provide a runnable Flutter/Dart toolchain, so no local formatter/analyzer/test/build result is claimed. Please rely on the repository's maintained CI and Platform Builds status for the exact PR commit.