Adds a `Compare ⇄` flow for diffing two K8s resources of the same kind.
Two entry points converge on the same view:

- **Drawer "Compare" button**: opens a picker (same-namespace promoted,
  alphabetical, ↑↓/Enter keyboard nav), navigates to `/compare?kind=&a=&b=`.
- **Table compare mode**: header toggle flips ResourcesView into pick
  mode — leading A/B badge column, sticky bottom tray with 2 slots,
  cap-at-2 with replace-oldest, Esc exits.

Diff view: Monaco DiffEditor (real impl of the existing `YamlDiffEditor`
stub), side-by-side or unified, hide-unchanged collapses regions, Spec-only
drops status. Per-side error rendering — failed side gets a red pill +
banner, working side still renders. Swap A↔B updates URL. Resources are
normalized before diffing (strip managedFields/uid/resourceVersion/last-
applied/pod-template-hash) so the diff is signal not noise.

Pure helpers extracted with tests: `parseRef`/`refToParam` (URL ref
parsing), `togglePick`/`pickIndex` (cap-replace state machine),
`sortCandidates`/`filterCandidates` (picker order), `normalizeForCompare`.
49 new tests pin the load-bearing behavior.

The frontend `useResources` hook now gates on `Boolean(kind)` so the
picker's lazy-on-open pattern doesn't fire a 404 for the empty kind.

Address findings from the second review pass:

- parseRef now rejects `?a=prod/` (empty name after slash) — was silently
  wedging callers in an indefinite loading state since useResource had
  nothing to fetch.
- CompareViewRoute no longer flashes "Failed to load side A" for a refetch
  failure that has cached data — banner now only fires when the side has
  no data at all. Stale beats misleading.
- Kind change in compare mode now also exits compare mode, not just clears
  picks — leaving the tray on with empty pills after the kind switch was
  the worst-of-both UX.
- Tray render now gated on `compareEnabled` (mirrors the existing toolbar
  toggle gate) so library consumers without onNavigate can't get a tray
  whose Compare CTA silently no-ops.
- Picker error prop typed `unknown` (was `Error | null`) — React Query
  emits unknown; renderer falls back through `String(err)` for non-Error
  throws.
- togglePick cap-replace rewritten as `[...picks.slice(1), ref]` — clearer
  intent than the old slice arithmetic that only happened to be correct
  for cap=2.

Type cleanup:
- Single `NamespacedRef` shape replaces the three accidental duplicates
  (Pick / CompareTrayPick / ParsedRef / SortableCandidate).
- `SIDE_TONES` const centralises the A/B palette used by the drawer pill,
  picker chip, tray pill, and table row badge. Palette changes touch
  one place instead of four.
- rowHighlightClass extracted from a 4-deep nested ternary in
  ResourceRowCells (CLAUDE.md flag).

Web cleanup:
- useCompareCandidates lifts the shared `useResources` + map pattern out
  of useCompareLauncher and CompareViewRoute.

Comments stripped (CLAUDE.md: no WHAT narration): A→B gradient ribbon,
"see the design memo" pointer, DNS-1123 duplication in url.test.ts,
verbose PINNING block in normalize.test.ts.

51 tests pass (+2 — empty-name URL rejection, slash-only URL rejection).

…tate

Five fixes from the Bugbot review on commit 54d8789:

- WorkloadView's compare launcher now prefers the URL-supplied `rest.group`
  over the apiVersion-derived one. Otherwise CRD compare clicked before
  the resource fetch completes (or after a fetch failure) loses the group
  and routes to the wrong kind on collisions.
- CompareResourcePicker now takes a `sourceSide` prop and renders the
  source chip with the matching SIDE_TONES color/letter. Clicking the
  pencil on the B pill no longer shows "A" for the current B resource;
  the header copy also switches to "Replace side B with another …".
- Picker resets `query` and `highlightIdx` when `open` transitions to
  true. The drawer flow keeps the picker mounted, so without this the
  previous session's search and highlight position leaked into the next
  compare.
- Picker now also clamps `highlightIdx` on query change (was only on
  filtered.length change). Typing in search could leave Enter selecting
  a row different from the visually highlighted one.
- Picker arrow keys no-op when the filtered list is empty. Previously
  ArrowDown on empty list set highlightIdx to -1.

The Bugbot "empty name passes URL validation" finding was already fixed
in commit 9c9e089 (parseRef rejects `?a=prod/`); the comment was stale.

Three fixes from Cursor Bugbot on commit 5805484:

- CRD compare was silently broken: App.tsx's URL-sync effect strips the
  bare `group` query param on every non-topology view (reserves it for
  topology grouping mode). Renamed the compare URL param `group` →
  `apiGroup` to match Radar's repo-wide convention. App.tsx never
  touches `apiGroup`, so the value survives navigation into /compare.
  Affected writers: useCompareLauncher, ResourcesView compare-nav,
  CompareViewRoute swap. Reader: CompareViewRoute. Docs updated.
- ResourceCompareView now takes per-side `aLoading` / `bLoading`
  instead of a single `loading` flag. A fast side renders immediately
  while the slow side spins; only both-pending shows the full-pane
  "Loading resources…" placeholder.
- When both sides fail, the error banner now lists both messages
  (`A: <aError> · B: <bError>`) instead of dropping bError on the floor.