feat: Add lattice diff command for branch-scoped change summaries

[gmoon]

Summary

• Adds lattice diff command that shows lattice nodes added, modified, resolved, or deleted since a given git ref
• Supports three output modes: colored text (default), JSON (--format json), and markdown (--md)
• Defaults to merge-base with main when no --since ref is provided

Implementation

• Core diff logic in src/diff.rs: uses git diff --name-status <ref> -- .lattice/ to detect file changes, parses YAML to extract node metadata, detects newly resolved requirements by comparing old vs new state
• CLI wiring in src/main.rs: new Diff command variant with --since, --md, and --format flags
• Library exports in src/lib.rs
• Lattice requirement REQ-CLI-019 and implementation IMP-CLI-019 added and verified

Test plan

• 10 unit tests covering is_node_file, DiffResult, format_entry, markdown formatting
• 8 integration tests: no-crash on HEAD, JSON structure, markdown output, changes since initial commit, explicit ref, invalid ref error, text output structure
• make pre-push passes (fmt + clippy + 208 tests + release build)
• lattice drift reports no unresolved drift

Closes #2

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