lore-proto, lore-revision, lore-server: opt-in last-commit attribution on RevisionTree

[lorelab-graham]

Summary

Adds TreeNode.last_commit to ThinClientService.RevisionTree, so a client rendering a directory listing can show per-entry last-commit message, timestamp, and revision identifier without walking history client-side. Off by default; existing callers pay nothing.

Without this, a client needs RevisionDiff per revision back from the tip plus RevisionInfo per touching revision to produce the same rendering.

Attribution rides on the per-entry back-pointer that lore-revision already maintains on NodeFileMetadata for file::history. One delta-block read per state, one metadata-block read per entry.

• New request field RevisionTreeRequest.include_last_commit gates the attribution work
• TreePath gains last_revision and last_revision_repository, populated inside enumerate_children from TreeAttribution built against that call's walk_state / walk_repository
• Attribution follows the walker across link boundaries. Each linked-subtree recursion builds its own TreeAttribution from the linked repository's state, so entries inside a link attribute against the linked repository's revisions rather than the parent's. NodeIDs are u32 indices - mixing states would silently return plausible garbage
• Directories inherit their descendant's revision through the existing metadata propagation, so folder rows attribute directly without a max-over-descendants pass
• Server handler deduplicates by (RepositoryId, Hash) and resolves each unique revision once via load_tree_commit, reusing the walked repository's context when possible and building a linked context only when the tree spans a link
• Legacy v0 handler passes include_last_commit: false - the v0 message has no field to carry attribution

Test Plan

• lore-revision unit tests: raw metadata invariant (per-entry back-pointer vs parent stamp), tree() end-to-end with attribution on and off, and cross-link attribution asserting the linked-subtree carries the linked repository's revision and id
• lore-server unit tests: load_tree_commit signature-and-identifier round-trip; include_last_commit gates the field without changing which nodes are emitted; per-entry attribution does not borrow a neighbour's commit and does not collide across repositories
• lore-proto shape test extended to destructure TreeCommit and the new fields on TreeNode / RevisionTreeRequest
• New end-to-end tests in scripts/test/test_revision_tree_last_commit.py cover the flag gate, per-entry attribution across two revisions, cross-link attribution, and directory propagation on the wire
• cargo test --workspace passes (no regressions)