[tinder-maxwellelliott]

What

Adopts serve --partialGraph (#586) in the one-shot CLI. generate-hashes --partialGraph hashes the working tree's own git tree object, records every rule's hashing recipe and the tree id in the file, and with --baseHashes <file> re-evaluates that base's recipes against the workspace's tree when git diff between the two trees only modifies regular file content: no bazel query, no Bazel at all. Anything else (graph-defining files, adds/deletes/renames, symlinks, repo-rule inputs, seed files, --partialGraphRebuildPattern, a base tree the clone lacks) falls back to a full generation with the reason logged. The file a derivation writes is byte-identical to the one a full generation in the mode writes, so derived files are bases for later runs.

Phases 1-2 of a four-phase plan; phases 3 (a CLI cache: --cacheDir, the shared S3/Nexus tiers, ancestor base search, --revision) and 4 (a CLI scenario in the nightly partial-graph bench) are not in this PR.

Why

The query is the whole cost of generate-hashes, and a source-only change cannot alter the graph. In the traditional two-file workflow this lets a PR job derive its hashes from the merge base's file without Bazel.

Decisions a reviewer should know about

• The revision is the working tree's tree object, built from a temporary copy of the index (add --all, write-tree), so the repo's own index is never touched. A clean tree yields HEAD^{tree}, so CLI and serve entries for the same commit share an id; a dirty tree yields a real tree object git diff/ls-tree accept. The workspace must be the repository root.
• The run's own output, --baseHashes and --depEdgesFile files are left out of the snapshot when they lie inside the workspace. Without this, a hashes file written into the repo shows up as an added path and forces a fallback on every run.
• Partial-graph files always carry target types, and the dependency edges when --depEdgesFile is given: recipes cannot be evaluated without kinds, and a derivation cannot invent edges (a base without them falls back when edges are requested). --targetType is rejected under the flag; filter in get-impacted-targets instead.
• get-impacted-targets and explain refuse a pair of files of which only one carries recipes, since the two formulas are not comparable.
• An unreadable --baseHashes is an error (a misconfiguration); a base without recipes or tree, or a classification failure, is a logged fallback.
• metadata.tree is written only in partial mode, so default-mode files and serve's default-mode cache entries are byte-for-byte unchanged.

Refactor

Git plumbing (revision/tree resolution, ls-tree indexing, the working-tree snapshot, checkout, classification) moves from server.rs to src/git.rs behind a Git runner that serve binds to the query's cancellation token; the configuration fingerprint and cache key move to src/cache.rs so a later CLI cache shares serve's <tree>.<fingerprint> key space. server.rs keeps one-line wrappers, so its tests did not move. The S3/Nexus tiers stay in server.rs until the CLI uses them.

Tests

• src/git.rs: resolution, snapshot (clean = HEAD tree, dirty edits, untracked/ignored, deletions, excluded files, no index file, non-root workspace), checkout with a stale index.lock, classification.
• src/cache.rs: fingerprint sensitivity, key shape.
• src/main.rs: flag parsing, every rejected combination, derive / same-tree / fallback paths against a real git repo with a never-called generator, the mixed-mode guard for both subcommands.
• e2e core::generate_hashes_partial_graph_derives_from_base_hashes: derived file byte-identical to a full generation, dep-edges files equal, same impacted set as the default mode, uncommitted and committed edits yield the same file, mixed modes refused, BUILD edit falls back.
• Verified locally: //src:rust_tests, //src:cli_tests, the new e2e case plus both serve partial-graph cases, //tools/e2e:regen_check, //:rust_format_check, clippy aspect. The coverage gate was not run locally.

README gains a "Partial Graph Updates" section (generated from tools/readme_template.md).

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