feat(llm): add replay token metering (PR2 round 1)

docs/core-data-structures/token-meter.md

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+# Token Meter
+
+`@deepseek-ai/dsh-token-meter` exposes detached replay measurements for request pressure and positional surface pricing. Scalar and surface snapshots carry the number of durable events consumed as `logRevision`; consumers compare revisions before making a joint decision.
+
+Source: [`packages/llm/token-meter/src/types.ts`](../../packages/llm/token-meter/src/types.ts)
+
+## `TokenMeasurement`
+
+```ts type-equiv
+interface TokenMeasurement {
+  /** Model profile used for every heuristic component. */
+  readonly model: string
+  /** Number of durable events consumed; equal to the next unread event seq. */
+  readonly logRevision: number
+  /** Provider or heuristic anchor used for this measurement. */
+  readonly baseline: TokenMeasurementBaseline
+  /** Signed repricing of current surface content relative to the baseline anchor. */
+  readonly surfaceDeltaTokens: number
+  /** Non-negative current request-and-response pressure. */
+  readonly totalTokens: number
+}
+```
+
+`baseline.kind === 'usage'` means a successful provider call has the same model and canonical envelope. `estimated` means the meter repriced the complete envelope and surface. Signed `surfaceDeltaTokens` preserves growth and shrinkage relative to a matching provider or estimated anchor.
+
+## `TokenSurfaceNode`
+
+```ts type-equiv
+interface TokenSurfaceNode {
+  /** Durable sequence number of the surface event. */
+  readonly seq: number
+  /** Heuristic tokens for the exact message projected by this node. */
+  readonly tokens: number
+}
+```
+
+## `TokenSurfaceMeasurement`
+
+```ts type-equiv
+interface TokenSurfaceMeasurement {
+  /** Model profile used to price every node. */
+  readonly model: string
+  /** Number of durable events consumed; equal to the next unread event seq. */
+  readonly logRevision: number
+  /** Total heuristic tokens across the current surface. */
+  readonly totalTokens: number
+  /** Current surface nodes in positional head-to-tail order. */
+  readonly nodes: readonly TokenSurfaceNode[]
+}
+```
+
+Surface order is authoritative; replacement nodes can have higher durable seqs than later positional nodes. The snapshot is immutable and does not grow when the underlying replay fold advances.