feat(web): add durable session metrics (round 1)

.agents/notes/implemented/architecture/2026-07-28-host-owned-web-session-metrics.i18n.yaml

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+# Bilingual-pair consistency record (docs/i18n/README.md): the git blob hash of each
+# side as of the last confirmed-consistent state. Both languages carry equal authority;
+# after editing either side, bring the other along and re-record with:
+#   pnpm run verify-translation-pairing --write .agents/notes/implemented/architecture/2026-07-28-host-owned-web-session-metrics.md
+2026-07-28-host-owned-web-session-metrics.md: 04381c7443491fd9de101a87713fa5c183800d0e
+2026-07-28-host-owned-web-session-metrics.zh.md: 6ad06ea61508d3f0703c19bc7c9f14969f119334

.agents/notes/implemented/architecture/2026-07-28-host-owned-web-session-metrics.md

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+# Agent Note: Host-owned Web session metrics
+
+Status: implemented
+
+English | [中文](2026-07-28-host-owned-web-session-metrics.zh.md)
+
+## Problem
+
+A Web stats line derived from the currently loaded conversation nodes is window-dependent under pagination. Compaction can replace visible content without preserving historical usage, and route changes leave the browser without an authoritative context capacity. Cache-write tokens also risk being folded into a cache-hit formula whose denominator has different semantics.
+
+## Decision
+
+The Host owns one session-level metrics projection. It incrementally folds the complete durable event log, keys settled usage by `(turn, step)`, and replaces an earlier usage record for the same key instead of double-counting chunk and message forms. Uncached input, output, cache reads, and cache writes remain four disjoint cumulative buckets. Compaction can change the current prompt surface without erasing historical usage.
+
+Current context pressure is a separate point-in-time value from `tokenMeter.measure(session).totalTokens`. Capacity comes only from `llm.resolveModelInfo(provider, model).context.contextWindow` for the agent's selected route. A route change immediately publishes metrics with capacity absent, then publishes the resolved capacity behind a route generation fence; stale metadata cannot label the new route.
+
+The tail `session.history` response carries the projection, while older pages omit it. Live changes use `session/metrics` mux frames. Both forms carry a durable-log revision and a projection revision; the client accepts only nondecreasing revisions, preserves metrics across older-page prepend, and clears them at a new subscription baseline. Missing measurement or metadata stays absent.
+
+The Web stats line treats the projection as its sole token source. It renders uncached input, output, and cache reads separately, computes cache hit as `cacheRead / (uncachedInput + cacheRead)`, and shows current context as a percentage of the exact route capacity. Cache writes never enter that percentage. Visible nodes continue to supply only turn and step counts.
+
+## Alternatives considered
+
+**Fold the loaded node window in React.** This cannot survive pagination or compaction and duplicates durable-log semantics in a presentation package.
+
+**Send usage only with raw assistant events.** Reconnect and older-page stitching would still need the client to reconstruct a full-log aggregate, and duplicate usage forms would need protocol-specific repair there.
+
+**Reuse one total-token field for cache hit.** Cache reads, cache writes, and uncached input represent distinct provider accounting buckets; combining them would make the displayed rate misleading.
+
+**Keep the previous capacity until the new route resolves.** The old number would temporarily claim the wrong selected model. An explicit unknown state is honest and generation-safe.
+
+## Consequences
+
+Token totals remain stable across pagination, replay, compaction, and browser reconnect. The client stores a small detached projection instead of scanning the conversation window, and the status row remains readable for large histories through compact number formatting.
+
+The Host performs one incremental log fold per session and schedules live projection updates only for usage, request-header, or surface-changing events; text and reasoning deltas do not publish metrics. Exact capacity resolution is asynchronous and may briefly render as unknown. Deployments without a token meter or model context metadata retain the row and label the unavailable value instead of fabricating one.

.agents/notes/implemented/architecture/2026-07-28-host-owned-web-session-metrics.zh.md

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+# Agent Note: Host 拥有的 Web 会话指标
+
+Status: implemented
+
+[English](2026-07-28-host-owned-web-session-metrics.md) | 中文
+
+## 问题
+
+Web 统计行若根据当前加载的会话节点推导指标，其结果会随分页窗口变化。压缩（compaction）可以替换可见内容，却无法保留历史用量；路由变更会让浏览器缺少权威的上下文容量。缓存写入 token 还可能被计入缓存命中率公式，而该公式的分母具有不同语义。
+
+## 决策
+
+Host 拥有一项会话级指标投影。它以增量方式归并完整的持久事件日志，按 `(turn, step)` 标识已结算用量；同一标识再次出现时，会替换较早的用量记录，而不会重复统计分片和消息两种形态。未缓存输入、输出、缓存读取与缓存写入保持为四个彼此独立的累计计数项。压缩可以改变当前提示词表层，但不会抹除历史用量。
+
+当前上下文压力是一个独立的即时值，取自 `tokenMeter.measure(session).totalTokens`。容量仅来自 `llm.resolveModelInfo(provider, model).context.contextWindow`，并对应 agent（智能体）所选的路由。路由变更时，Host 会立即发布不带容量的指标，再通过路由代际围栏发布解析出的容量；陈旧元数据无法标记新的路由。
+
+`session.history` 尾页响应携带该投影，较早页面则省略它。实时变更使用 `session/metrics` mux 帧。两种形式都携带持久日志修订号和投影修订号；客户端只接受不减小的修订号，在向前加载较早页面时保留指标，并在建立新的订阅基线时将其清除。测量值或元数据缺失时，对应字段保持缺失。
+
+Web 统计行把该投影视为唯一的 token 数据来源。它分别呈现未缓存输入、输出与缓存读取，通过 `cacheRead / (uncachedInput + cacheRead)` 计算缓存命中率，并把当前上下文显示为精确路由容量的百分比。缓存写入绝不计入缓存命中率。可见节点仍然只提供轮次和步骤计数。
+
+## 备选方案
+
+**在 React 中归并已加载的节点窗口。** 此方案无法跨越分页或压缩保留数据，还会在展示包中重复实现持久日志语义。
+
+**只随原始 assistant 事件发送用量。** 重连和较早页面拼接仍会要求客户端重建完整日志聚合，而且重复的用量形态需要在客户端按协议专门修复。
+
+**为缓存命中率复用单一的 token 总数字段。** 缓存读取、缓存写入与未缓存输入是提供方记账中的不同计数项；将它们合并会使显示的比率产生误导。
+
+**在新路由解析完成前保留旧容量。** 旧数值会在短时间内错误标示所选模型。显式的「未知」状态能如实反映情况，并避免跨代串扰。
+
+## 后果
+
+token 总量在分页、回放、压缩和浏览器重连期间保持稳定。客户端存储一项小型脱耦投影，无需扫描会话窗口；状态行采用紧凑数字格式，因此在较长的历史记录中仍然清晰易读。
+
+Host 为每个会话执行一次增量日志归并，仅为用量事件、请求头事件或表层变更事件调度实时投影更新；文本与推理（reasoning）增量不会发布指标。精确容量解析为异步操作，因此可能短暂显示「未知」。未部署 token 计量器或缺少模型上下文元数据时，系统仍保留该行，并标示不可用的值，而不会虚构数据。