feat: compute target distances in bazel-diff serve

[tinder-maxwellelliott]

What

Adds build-graph target-distance computation to the bazel-diff serve HTTP query service (issue #29 follow-up). The distance math already existed for the CLI (get-impacted-targets --depEdgesFile); this wires it into the server.

New endpoint

GET /impacted_targets_with_distances?from=<rev>&to=<rev>[&targetType=...] — like /impacted_targets, but each impacted target is annotated with targetDistance (hops to the nearest directly-changed target) and packageDistance (how many of those hops cross a package boundary). Directly-changed targets sit at distance 0.

{
  "from": "9a1c0e2…",
  "to": "3f7b8d4…",
  "impactedTargets": [
    {"label": "//foo:bar", "targetDistance": 0, "packageDistance": 0},
    {"label": "//foo:baz", "targetDistance": 1, "packageDistance": 1}
  ]
}

Opt-in flag

serve --trackDeps (default off) enables dependency-edge tracking. Tracking deps grows each cached hash entry, so it's opt-in. The flag is folded into the cache fingerprint, so a deps-tracking server never reuses a deps-less cache entry (or vice-versa). The distances endpoint returns 400 when the server was started without --trackDeps.

How

• CalculateImpactedTargetsInteractor — extracted computeImpactedTargetsWithDistances() returning structured ImpactedTargetWithDistance data; executeWithDistances delegates to it, so the CLI's JSON output stays byte-identical.
• HashService / HashFileData / DeserialiseHashesInteractor — the per-SHA cache persists the dep-edge adjacency list under metadata.depEdges and round-trips it on a cache hit.
• ImpactedTargetsService — new distance-aware method using the to-revision graph, reusing the existing withWorkspaceAt module-changed locking.
• BazelDiffServer — new handler; shared GET/readiness/param parsing factored into one helper.
• ServeCommand — --trackDeps flag, wiring, and fingerprint inclusion.

Tests

• New/extended unit coverage: CalculateImpactedTargetsInteractorTest, DeserialiseHashesInteractorTest, HashServiceTest, ImpactedTargetsServiceTest, BazelDiffServerTest, ServeCommandTest.
• E2E testServeEndToEnd extended with --trackDeps + the new endpoint (asserts //:lib at distance 0).
• bazel test //cli/... — all unit suites pass; testServeEndToEnd and testTargetDistanceMetrics pass. (Unrelated cquery/external-repo E2E tests fail only due to the local sandbox's missing JDK/coursier.)
• README regenerated; --trackDeps flows into the serve --help block.

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