Add diff --format json|toon and polish CLI help

[jantimon]

Implements the coordinator-approved set from the CLI help/UX review.

diff --format json|toon

diff now serializes the same comparison the human report shows, so an agent can consume it structurally (AGENTS.md's blanket "always pass --format json" rule now holds for diff too). Human output and --fail-on-regression exit codes are unchanged in both formats. The computation is factored into a pure buildDiffView (no printing, no process.exitCode); a thin renderer prints the human report and the emitter serializes.

The emitted view (model/query.ts, kept off the stored types like every verb):

• DiffView: { baseline, current, comparability[], metrics[], sliceDeltas, regressions[], gateRefusal?, failOnRegression, failed }.
  • metrics[] = DiffMetricRow { key, label, gated, base, current, delta, regression } — base/current/delta keep the Measured null (never a fabricated 0), regression true only for a gated count that worsened.
  • comparability[] = DiffComparabilityAxis { axis, base, current, blocksGating } (the comparabilityMismatches axes).
  • sliceDeltas reuses the existing SpanSliceDiff (advisory per-span slice ms).
  • gateRefusal = why a --fail-on-regression gate could not be evaluated (incompatible capture, or known-incomplete counts); failed = the process verdict (exit 1).
• GroupDiffView: { baseline, current, refusal?, members[], failOnRegression, failed } for a run-group diff, each member carrying its per-pair DiffView (or onlyIn). DiffOutput = DiffView | GroupDiffView.

New public exports: DiffView, DiffMetricRow, DiffMetricKey, DiffComparabilityAxis, GroupDiffView, GroupDiffMember, DiffOutput, plus the transitively-referenced SpanSliceDiff/SliceDelta/SliceName (the export-completeness unit test requires them).

CLI help / UX

• Tagline now names layout/paint/style/invalidation and CPU self-time, and all three targets (Chrome/Firefox/Node), instead of Chrome-only.
• --help gains a quick-start epilog (record --url + query spans latest); record --help gains a "read it with query spans latest" epilog.
• query events --kind lists the missing gc (the EventKind union has 10 kinds; help listed 9).
• query get uses the shared fmtOpts so the hidden --json alias works there too.
• query blame --forced help em-dash -> ASCII --.
• --breakdown/--deep help each note their mutual exclusivity ("for both, record a run group via --members") and a see-also tail matching --alloc ("Read it with query spans / query blame --forced").
• AGENTS.md lists DiffView in the exported view shapes.

Verification

Full gate batch under the shared gate-lock: build, lint, format:check, knip, 686 unit, 6 measurement, 51 chrome e2e — all green. Manually verified diff --format json/toon/--json output and that --fail-on-regression exit codes match between human and structured output. Self-review: structured hostile two-pass (the review-intern-sub subagent-spawn tool was not available in this worktree); caught and fixed one archeology phrase in a doc comment.