Add assert --format json|toon (typed AssertView)

[jantimon]

Problem

assert emitted only a human ASCII table plus an exit code. diff and cpu-diff both have --format json, but a CI PR-comment script gating on assert had to scrape the ASCII table to render "layout slice 6 ms > 4 ms budget" — the one place in a gate that consumed text instead of structured output.

Change

assert gains --format json|toon, emitting a typed AssertView (additive, off the stored types, the same pattern as the other views):

• thresholds[]: one AssertThresholdRow per gate — target (run / step / group), metric, axis (count | timing | slice), budget, value (the measured number or null when not measured), verdict (pass | fail | n/a-fail), and the routed member on a run-group.
• passed, violations[], notes[] (group disclosures), and a kind discriminator (recording | group).

Example (--target node, a not-measured forced-count on a node recording is a loud n/a-fail):

{
  "target": ".../rec.json", "kind": "recording",
  "thresholds": [
    { "target": "run", "metric": "forced layout/style", "axis": "count",
      "budget": 0, "value": null, "verdict": "n/a-fail", "reason": "not measured" },
    { "target": "run", "metric": "wall ms", "axis": "timing",
      "budget": 100000, "value": 22.565, "verdict": "pass" }
  ],
  "passed": false,
  "violations": ["run: forced layout/style was not measured; cannot satisfy max 0"],
  "notes": []
}

Invariants held

• Exit code unchanged: passed is true exactly when the process exits 0. A --format json gate fails the build exactly as the human report does (unit + e2e assert passed↔status).
• Human output byte-identical: the table and its wording are untouched; JSON is an early branch before the table code.
• The n/a-fail contract: a not-measured metric is value: null / verdict: "n/a-fail" / exit 1, never a fabricated 0 that silently passes.

Scope

• src/model/query.ts: AssertView, AssertThresholdRow, AssertVerdict, AssertAxis.
• src/commands/assert.ts: build the view in both the plain and run-group paths; a shared finishAssert emits structured or prints the human report and sets the exit code.
• src/cli.ts: --format + hidden --json on assert.
• src/index.ts + README type table + public-types walker: AssertView/AssertThresholdRow exported and enumerated.
• New test/unit/assert-format.test.mjs (7 tests: shape, the n/a-fail exit-code guarantee, the slice axis, TOON, exit-code parity, and the run-group member routing).

Gates: lint, format:check, build, unit (745), the 3 Chrome-driven assert e2e tests all green.