feat(format): add deterministic stats output

[aram-devdocs]

Fixes #46

Summary

• add deterministic formatter stats to pretty and JSON output, including severity counts, distinct viewport count, distinct rule count, and a content-hashed run_id
• group pretty output by viewport, then rule, then selector while keeping JSON ordering canonical by violation sort key
• add snapshot and determinism coverage for grouped pretty output and JSON stats, and document the expanded MCP full-detail envelope

Tests Run

• cargo test -p plumb-format
• cargo fmt --check
• cargo check --no-default-features

Coordination

• feat(format): SARIF 2.1.0 with rule metadata + helpUri #44: stats and grouped pretty formatting stay within plumb-format and do not depend on SARIF internals