Expose buildDiff in /eval/:id?compare=X JSON response

[luochen1990]

Motivation

Tools that track channel health (e.g. deciding whether bumping a NixOS host's `nixpkgs` input to a newer eval is safe) need to know which builds changed status between two evals. The diff is already computed server-side, but currently only reachable via the HTML view — which truncates each category to 250 entries and requires parsing HTML tables.

This exposes the existing `buildDiff` result in the JSON response so API clients can consume it directly.

What changes

`GET /eval/:id?compare=X` with `Accept: application/json` now includes a `diff` field alongside the existing eval fields:

Request	Before	After
`/eval/:id` (no compare) JSON	eval fields + build IDs	unchanged
`/eval/:id?compare=X` JSON	eval fields + build IDs	+ `diff` field
`/eval/:id` HTML	unchanged	unchanged

The `diff` field is gated on the explicit `compare` query param (not on whether a previous eval exists). Without `compare`, the JSON response is unchanged — the controller's default behavior of auto-resolving a previous eval for the HTML diff tabs does not leak into the JSON entity.

Per-build field set: `{id, job}`

Each build in the diff is summarized to `{id, job}` — the minimal set that lets consumers:

• `job`: identify the attrpath + system (e.g. `nixpkgs.python3.x86_64-linux` → attrpath `python3` by stripping the arch suffix)
• `id`: link to `/build/{id}` for details (no bulk job→id API exists in hydra, so omitting `id` would force consumers back to per-build queries)

The `removed` category has a reduced shape `{job, system}` since those jobs don't exist in the current eval (no `id`).

Example

```bash
curl -H 'Accept: application/json' \
'https://hydra.nixos.org/eval/1827224?compare=1827222'
```

```json
{
"id": 1827224,
"builds": [336650910],
"diff": {
"stillSucceed": [
{"id": 336650910, "job": "tests/api.x86_64-linux"}
],
"stillFail": [],
"nowSucceed": [],
"nowFail": [],
"new": [],
"aborted": [],
"unfinished": [],
"removed": [],
"totalAborted": 0,
"totalFailed": 0,
"totalQueued": 0
}
}
```

Unlike the HTML view, the `diff` field is not truncated — `stillFail` returns every failing build, matching what `buildDiff` (`src/lib/Hydra/Helper/BuildDiff.pm`) already computes.

Testing

New test `t/Hydra/Controller/JobsetEval/diff.t`:

• without `compare`: no `diff` field (regression guard — the controller auto-resolves eval2 even without compare, so naive gating on eval2 would leak)
• with `compare`: `diff` field present with all 11 expected keys
• each build has exactly `{id, job}`; `removed` has `{job, system}`

Full suite (`meson test` + perlcritic severity=1): 99 files, 1558 assertions, 0 failures.