feat(web): turn speed metrics and composer context meter

Assistant footers and the stats line gain TTFT/tok-per-second readings
folded from step timings; context occupancy moves off the stats line onto
a composer ring whose panel shows a heuristic system/tools/messages
breakdown from the new token-meter contextBreakdown session projection.

packages/llm/token-meter/src/estimate.ts

@@ -0,0 +1,87 @@
+/**
+ * Fixed-density heuristic token pricing shared by the meter service and the
+ * pure context-breakdown projection, so both surfaces price identical content
+ * to identical numbers.
+ *
+ * @module @deepseek-ai/dsh-token-meter/estimate
+ */
+
+import type { ContentBlock, Message } from '@deepseek-ai/dsh-llm'
+import type { EpochHeader } from '@deepseek-ai/dsh-session'
+
+/** Fixed text-density estimate used until exact tokenization is needed. */
+const CHARS_PER_TOKEN = 4
+
+/** Per-block structural overhead for JSON framing and type tags. */
+const BLOCK_OVERHEAD = 4
+
+/** Role-field framing overhead added to every priced message. */
+export const ROLE_OVERHEAD = 4
+
+/**
+ * Price content blocks recursively under the fixed density heuristic.
+ * @param blocks - content blocks to price without mutation.
+ * @returns heuristic tokens including per-block structural overhead.
+ */
+export function estimateContent(blocks: readonly ContentBlock[]): number {
+  let tokens = 0
+  for (const block of blocks) {
+    switch (block.type) {
+      case 'text':
+      case 'reasoning':
+        tokens += Math.ceil(block.text.length / CHARS_PER_TOKEN) + BLOCK_OVERHEAD
+        break
+      case 'tool-call':
+        tokens += Math.ceil(block.name.length / CHARS_PER_TOKEN)
+          + Math.ceil(block.arguments.length / CHARS_PER_TOKEN)
+          + BLOCK_OVERHEAD
+        break
+      case 'tool-result':
+        tokens += estimateContent(block.content) + BLOCK_OVERHEAD
+        break
+      default:
+        // ContentBlockMap is merge-extensible; unknown blocks retain a
+        // conservative structural JSON price under the fixed heuristic.
+        tokens += BLOCK_OVERHEAD + Math.ceil(JSON.stringify(block).length / CHARS_PER_TOKEN)
+    }
+  }
+  return tokens
+}
+
+/**
+ * Heuristically price one model-visible message.
+ * @param message - message to price without mutation.
+ * @returns content and role-framing tokens under the fixed heuristic.
+ */
+export function estimateMessage(message: Message): number {
+  return estimateContent(message.content) + ROLE_OVERHEAD
+}
+
+/**
+ * Price the system-prompt part of a canonical request envelope.
+ * @param header - canonical envelope, or undefined before any request.
+ * @returns heuristic system-prompt tokens; 0 when absent.
+ */
+export function estimateSystemTokens(header: EpochHeader | undefined): number {
+  if (header?.system === undefined) return 0
+  return Math.ceil(header.system.length / CHARS_PER_TOKEN) + ROLE_OVERHEAD
+}
+
+/**
+ * Price the tool-schema part of a canonical request envelope.
+ * @param header - canonical envelope, or undefined before any request.
+ * @returns heuristic tool-schema tokens; 0 when absent or empty.
+ */
+export function estimateToolsTokens(header: EpochHeader | undefined): number {
+  if (header?.tools === undefined || header.tools.length === 0) return 0
+  return Math.ceil(JSON.stringify(header.tools).length / CHARS_PER_TOKEN) + BLOCK_OVERHEAD
+}
+
+/**
+ * Price the complete non-surface request envelope.
+ * @param header - canonical envelope, or undefined before any request.
+ * @returns heuristic system plus tool tokens.
+ */
+export function estimateHeader(header: EpochHeader | undefined): number {
+  return estimateSystemTokens(header) + estimateToolsTokens(header)
+}