feat: show per-step OpenRouter cost on the Trajectory tab

Extend the openRouterCost projection with a per-step priced-cost map
keyed by turn:step so surfaces can render spend without re-pricing, and
have the Trajectory view read it through the framework useProjection seat:
each priced assistant step shows its USD cost on the request boundary chip
and in the inspector summary, while unpriced steps stay blank. Also fix a
pre-existing exactOptionalPropertyTypes error in extractCacheRates that the
host typecheck surfaced.

packages/llm/openrouter-usage/tests/projection.spec.ts

@@ -27,7 +27,7 @@ async function harness(): Promise<{ ctx: Context; session: Session }> {
   await ctx.plugin(SessionStore)
   await ctx.plugin(SessionProjectionRegistry)
   const session = ctx.sessions.create()
-  ctx.sessionProjections.register(createOpenRouterCostProjection((model) => PRICING.get(model)))
+  ctx.sessionProjections.register(createOpenRouterCostProjection(model => PRICING.get(model)))
   return { ctx, session }
 }
 
@@ -75,7 +75,7 @@ const projected = (ctx: Context, session: Session) => {
 describe('openRouterCost session projection', () => {
   it('serves an all-zero view on an empty log', async () => {
     const { ctx, session } = await harness()
-    expect(projected(ctx, session)).toEqual({ totalUsd: 0, pricedSteps: 0, unknownModelSteps: 0, currency: 'USD' })
+    expect(projected(ctx, session)).toEqual({ totalUsd: 0, pricedSteps: 0, unknownModelSteps: 0, steps: {}, currency: 'USD' })
   })
 
   it('prices input/output/cache buckets at the model rates', async () => {
@@ -98,7 +98,13 @@ describe('openRouterCost session projection', () => {
     startStep(session, 1, 1)
     const source = usageChunk(session, { inputTokens: 10, outputTokens: 4 }, 1, 1)
     finalUsage(session, { inputTokens: 10, outputTokens: 4 }, 1, 1, [source])
-    expect(projected(ctx, session)).toEqual({ totalUsd: 10 * 1.4e-6 + 4 * 2.8e-6, pricedSteps: 1, unknownModelSteps: 0, currency: 'USD' })
+    expect(projected(ctx, session)).toEqual({
+      totalUsd: 10 * 1.4e-6 + 4 * 2.8e-6,
+      pricedSteps: 1,
+      unknownModelSteps: 0,
+      steps: { '1:1': 10 * 1.4e-6 + 4 * 2.8e-6 },
+      currency: 'USD',
+    })
   })
 
   it('replaces an earlier same-step chunk sample with the final usage', async () => {
@@ -111,6 +117,7 @@ describe('openRouterCost session projection', () => {
       totalUsd: 14 * 1.4e-6 + 5 * 2.8e-6,
       pricedSteps: 1,
       unknownModelSteps: 0,
+      steps: { '1:1': 14 * 1.4e-6 + 5 * 2.8e-6 },
       currency: 'USD',
     })
   })
@@ -123,6 +130,7 @@ describe('openRouterCost session projection', () => {
     session.append('step/end', { turn: 1, step: 1 })
     expect(projected(ctx, session).totalUsd).toBeCloseTo(9 * 1.4e-6 + 1 * 2.8e-6, 12)
     expect(projected(ctx, session).pricedSteps).toBe(1)
+    expect(projected(ctx, session).steps).toEqual({ '1:1': 9 * 1.4e-6 + 1 * 2.8e-6 })
   })
 
   it('counts an unknown-priced OpenRouter model as an unpriced step', async () => {
@@ -131,7 +139,7 @@ describe('openRouterCost session projection', () => {
     startStep(session, 1, 1)
     usageChunk(session, { inputTokens: 100, outputTokens: 1 }, 1, 1)
     session.append('step/end', { turn: 1, step: 1 })
-    expect(projected(ctx, session)).toEqual({ totalUsd: 0, pricedSteps: 0, unknownModelSteps: 1, currency: 'USD' })
+    expect(projected(ctx, session)).toEqual({ totalUsd: 0, pricedSteps: 0, unknownModelSteps: 1, steps: {}, currency: 'USD' })
   })
 
   it('ignores a step on a non-openrouter provider entirely', async () => {
@@ -140,7 +148,7 @@ describe('openRouterCost session projection', () => {
     startStep(session, 1, 1)
     usageChunk(session, { inputTokens: 100, outputTokens: 1 }, 1, 1)
     session.append('step/end', { turn: 1, step: 1 })
-    expect(projected(ctx, session)).toEqual({ totalUsd: 0, pricedSteps: 0, unknownModelSteps: 0, currency: 'USD' })
+    expect(projected(ctx, session)).toEqual({ totalUsd: 0, pricedSteps: 0, unknownModelSteps: 0, steps: {}, currency: 'USD' })
   })
 
   it('prefers the assistant-message source over the last request/context record', async () => {
@@ -172,7 +180,48 @@ describe('openRouterCost session projection', () => {
     startStep(session, 1, 1)
     usageChunk(session, { inputTokens: 100, outputTokens: 10 }, 1, 1)
     session.append('step/end', { turn: 1, step: 1 })
-    expect(projected(ctx, session)).toEqual({ totalUsd: 0, pricedSteps: 1, unknownModelSteps: 0, currency: 'USD' })
+    expect(projected(ctx, session)).toEqual({ totalUsd: 0, pricedSteps: 1, unknownModelSteps: 0, steps: { '1:1': 0 }, currency: 'USD' })
+  })
+
+  it('maps each step to its priced cost under turn:step keys', async () => {
+    const { ctx, session } = await harness()
+    recordContext(session)
+    startStep(session, 1, 1)
+    usageChunk(session, { inputTokens: 1_000, outputTokens: 500 }, 1, 1)
+    startStep(session, 1, 2)
+    usageChunk(session, { inputTokens: 200, outputTokens: 100 }, 1, 2)
+    session.append('step/end', { turn: 1, step: 2 })
+    expect(projected(ctx, session)).toEqual({
+      totalUsd: (1_000 * 1.4e-6 + 500 * 2.8e-6) + (200 * 1.4e-6 + 100 * 2.8e-6),
+      pricedSteps: 2,
+      unknownModelSteps: 0,
+      steps: {
+        '1:1': 1_000 * 1.4e-6 + 500 * 2.8e-6,
+        '1:2': 200 * 1.4e-6 + 100 * 2.8e-6,
+      },
+      currency: 'USD',
+    })
+  })
+
+  it('drops a step entry when the final sample re-attributes it to an unknown-priced model', async () => {
+    const { ctx, session } = await harness()
+    recordContext(session)
+    startStep(session, 1, 1)
+    const source = usageChunk(session, { inputTokens: 100, outputTokens: 10 }, 1, 1)
+    session.append('assistant/message', {
+      turn: 1,
+      step: 1,
+      message: createMessage({
+        role: 'assistant',
+        content: [],
+        source: { kind: 'model', provider: 'openrouter', model: 'brand-new/model' },
+      }),
+      usage: { inputTokens: 100, outputTokens: 10 },
+    }, { surfaceOp: 'append', sourceEventSeqs: [source] })
+    session.append('step/end', { turn: 1, step: 1 })
+    // The earlier priced chunk's entry is removed with the re-attribution: the
+    // step is no longer priced, so it has no entry.
+    expect(projected(ctx, session)).toEqual({ totalUsd: 0, pricedSteps: 0, unknownModelSteps: 1, steps: {}, currency: 'USD' })
   })
 
   it('pushes no change for unrelated events', async () => {
@@ -199,6 +248,7 @@ describe('openRouterCost session projection', () => {
       totalUsd: 8 * 1.4e-6 + 2 * 2.8e-6,
       pricedSteps: 1,
       unknownModelSteps: 0,
+      steps: { '1:1': 8 * 1.4e-6 + 2 * 2.8e-6 },
       currency: 'USD',
     })
   })