feat(desktop): trace signal detection + tri-view badges

Adds a small signal-detection layer over trace payloads and surfaces
three kinds of semantic signals inline in the trace tri-view (Timeline
/ Graph / turn footer chips) so a researcher can spot repetitive
tool-call loops, redundant identical calls, and plan-completion beats
without hand-scanning the raw log:

- loop-detected  — three or more repeats of the same tool.method
                   signature inside a window, badge in Timeline gutter
                   + colored ring on the Graph node
- redundant-call — identical (tool, arg-hash) call twice without any
                   state change in between, badge + subdued styling
                   so it stays advisory not alarmist
- plan-* chips   — plan/subplan step-complete beats surfaced above the
                   assistant body in the turn footer, wired through
                   the finishTurnContainer tail

Pure rendering; no wire-format changes needed on the runtime side —
signal detection is fixture-driven off the same payload the tri-view
already consumes. Companion RFC L-2 in docs/upstream-ledger.md asks
the runtime to emit these signals natively rather than deriving them
in the shell, so once L-2 lands the shell will consume upstream signals
and this detector becomes a fallback.

Files:
  src/renderer/trace-signal-detect.js       new  (loop/redundant/plan)
  src/renderer/trace-tri-view.js            +24  (wire detector output)
  src/renderer/trace-timeline.js            +39  (badge in gutter)
  src/renderer/trace-graph.js               +30  (colored ring)
  src/renderer/renderer.js                  +81  (finishTurnContainer chips)
  src/renderer/index.html                    +1  (one script tag)
  src/renderer/style.css                    +58  (badge/ring/chip rules)
  docs/upstream-ledger.md                  +121  (append L-2 RFC)
  scripts/qa-trace-signals-fixture.mjs     new  (headless SVG proof)
  scripts/qa-trace-signals-shoot.mjs       new  (CDP live shoot)
  docs/trace-signals-shoot/*.html          new  (3 fixture-driven shots)
  test/trace-signal-detect.test.js         new  (detector unit tests)
  test/trace-signal-overlay.test.js        new  (renderer overlay tests)

Test suite: 1668/1668 pass (17 new). Fixture-driven signals-01-timeline-
loop.html regenerated byte-identical from the merged HEAD.

examples/desktop/src/renderer/trace-graph.js

@@ -372,6 +372,36 @@ function renderGraph(doc, input, opts) {
       class: `trace-graph-node-body family-${node.family}`,
     })
     nodeG.appendChild(circle)
+
+    // Signal ring — a colored outer stroke when this node's seq shows up
+    // in the passed signals map. Multiple signals share one ring but the
+    // tooltip enumerates them. See trace-signal-detect.js + L-2 RFC.
+    if (options.signals && typeof options.signals.get === 'function' && node.seq !== null) {
+      const sigs = options.signals.get(node.seq)
+      if (sigs && sigs.length) {
+        // Highest-priority signal wins the ring class (error > loop > redundant > plan).
+        const priority = ['tool-error', 'loop-detected', 'plan-restart', 'redundant-call', 'plan-update']
+        let winner = sigs[0]
+        let winnerRank = 999
+        for (const s of sigs) {
+          const r = priority.indexOf(s.signal)
+          if (r >= 0 && r < winnerRank) { winner = s; winnerRank = r }
+        }
+        const SD = (typeof window !== 'undefined' && window.__dshTraceSignalDetect)
+          || (typeof require !== 'undefined' ? require('./trace-signal-detect.js') : null)
+        const cls = SD ? SD.classFor(winner.signal) : 'sig-generic'
+        const ring = svgEl(doc, 'circle', {
+          cx: 0, cy: 0, r: NODE_R + 4,
+          class: `trace-graph-signal-ring ${cls}`,
+          fill: 'none',
+        })
+        const title = svgEl(doc, 'title', {})
+        title.textContent = SD ? sigs.map(s => SD.tooltipFor(s)).join('\n') : sigs.map(s => s.signal).join(', ')
+        ring.appendChild(title)
+        nodeG.appendChild(ring)
+      }
+    }
+
     const glyph = svgEl(doc, 'text', {
       x: 0, y: 4, class: 'trace-graph-node-glyph',
       'text-anchor': 'middle',