feat: semantic lineage diff across Git refs (Bundle 8) - #12

Summary

Bundle 8 of the lakehouse-native stack, based on Bundle 7 (#11). lineage --diff <git-ref> is the code-side complement to the data-side branch_diff: it answers "what does this branch change about the graph itself" before promote merges the data branch.

• core/lineage_diff.rs: lineage_diff(base, candidate) compares two LineageGraphs node-for-node and edge-for-edge — nodes_added/removed/changed (version, materialization, target, type, path), edges_added/removed (dependency structure — a model that started reading a dataset is a changed input edge, not just a changed file), edges_changed (column derives edges whose transformation/directness/confidence/expression moved), and impacts naming the base-side consumers each removed node orphans.
• core/git.rs::checkout_tree: materialises the workspace subtree at a resolved commit into a tempdir via ls-tree + cat-file --batch — read-only, no worktree, safe beside a dirty checkout or in-progress rebase. Handles nested workspace prefixes and symlinked files.
• lineage --diff <ref>: loads + compiles the base tree, applies the same --catalog override and catalogue enrichment as the candidate compile (so enriched source columns do not diff as "added"), prints the delta, writes .phlo/transform/lineage_diff.json with the resolved base_commit, and fails cleanly when the base ref cannot load or compile rather than diffing a partial graph.
• NodeMeta gains PartialEq for field-level comparison.

In the WAP flow the two reviews now sit side by side: lineage --diff proves the definitions, branch_diff proves the data.

Test plan

• cargo test --workspace — all suites green
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo fmt --all -- --check — clean
• 6 unit tests in lineage_diff.rs (empty diff, added/removed, version change, new dependency edge, orphan impact, column-lineage change)
• 4 CLI tests against a real git repo (clean diff, removed model + orphans, changed model JSON, artifact written)

Stacked on #11 — do not merge independently.