Introduce the Hebrus engine compatibility bridge

[andreaborio]

Outcome

Introduces the Hebrus public engine identity while preserving DS4 compatibility, serialized model formats, existing environment variables, and fork history.

What changed

• adds canonical hebrus* commands with tested ds4* aliases
• exposes a versioned, model-free capability contract for Studio integrations
• adds the compatibility ADR, migration guide, rollout plan, attribution, governance, security, and community documentation
• hardens install/uninstall, CI, release evidence, brand-boundary auditing, and server alias parity
• reconciles the rebrand with the current Qwen Metal FlashAttention and SSD mainline

Compatibility

The bridge retains ds4.expert_major.v2, published GGUF identities and hashes, disk-KV data, DS4_* variables, historical links, authorship, and the fork relationship to antirez/ds4. DS4 command aliases remain supported through the complete 1.x line.

Validation

• make premerge
• Metal and CPU builds
• capability and five command-alias pairs on both backends
• model-free server, Metal kernel, ExpertMajor, Qwen, install/uninstall, documentation, context, brand, artifact, and release-contract gates

Model-backed release qualification remains intentionally outside this PR's model-free evidence and is required before a final release.