Wire mutation/evidence validator and fixture generators into devtools

[mdheller]

Purpose

Implement the developer tooling side of SourceOS Mutation and Evidence Accountability, anchored to SourceOS spec PR #96:

SourceOS-Linux/sourceos-spec#96

Required work

• Package and expose the mutation/evidence validator for local development.
• Add fixture generators for browser write pressure, delegated sync, archive extraction, diagnostic self-noise, evidence routing gaps, and degraded-sensor clearance attempts.
• Add CI helpers that validate examples and reject anti-pattern fixtures.
• Provide a command such as sourceos-devtools validate mutation-evidence or equivalent.

Acceptance criteria

• The validator can be run locally against the schema bundles from sourceos-spec.
• Fixture generation produces both valid and invalid examples.
• CI fails if an anti-pattern becomes valid.
• Devtools documentation links back to PR #96 and the canonical spec location.