feat(core): record title and compaction usage in session totals - #37441

Problem

Title generation and compaction make billable LLM requests, but their reported tokens and cost were discarded. Session totals only included normal assistant steps. (V1 counted compaction usage through step-finish parts but silently dropped title usage; V2 dropped both.)

Implementation

• Extracted shared token normalization and cost calculation into session/usage.ts (SessionUsage.tokens, calculateCost, record, add), deduplicating the copies in runner/llm.ts and publish-llm-event.ts.
• Captured provider usage from the title and compaction streams, accumulating across step-finish events with per-step tier costing, and flushing on both completion and interrupt.
• Added a durable session.usage.recorded event for side inferences, mirroring how session.step.ended carries usage for normal steps. Durability is required: session totals are a projection of the durable log and are rebuilt through replay/sync, so a direct write would be lost.
• Projected the event into session token/cost totals via the existing applyUsage fold and wired it into the aggregate session.usage.updated broadcast.
• Kept the event internal-durable: it is in SessionEvent.DurableDefinitions (replay/sync manifest and the experimental durable session log contract) but excluded from SessionEvent.Definitions, so it never reaches the public event stream or manifest. Clients observe usage only through the aggregate session.usage.updated.
• Regenerated public client types: SessionUsageRecorded is absent from V2Event and present only in the durable-log SessionEventDurable union.

Testing

• Title and compaction tests now assert projected session totals, cost, and durable event ordering (Started → UsageRecorded → Ended).
• Manifest test pins the internal/public split: present in EventManifest.Durable, absent from Definitions/ServerDefinitions/Latest.
• Ran core session/event/log suites, full schema suite, and typechecks across core, schema, protocol, client, and server.