Summary

Fixes #3591 by serializing access to session title, token, and cost fields through Session.mu.

• adds locked title and token/cost setters with matching snapshot accessors
• routes runtime, store, server, app, evaluation, and TUI call sites through those accessors
• snapshots message usage data before rendering the cost dialog
• adds race regressions for session accessors, in-memory store updates, API responses, and TUI cost rendering
• preserves omitted fields when importing partial token usage events

Issue expectations

Validation

• task build
• task lint
• env -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u http_proxy -u https_proxy -u all_proxy task test
• go test -race -count=1 ./pkg/session/... ./pkg/server/... ./pkg/runtime/... ./pkg/app/... ./pkg/tui/dialog/...
• go test -race -count=1 ./pkg/a2a/... ./pkg/acp/... ./pkg/cli/... ./pkg/tui/page/chat/... ./pkg/chatserver/...

The full evaluation race suite still reports the pre-existing stderrData race in pkg/evaluation/eval.go; the evaluation tests affected by this change pass, and the ordinary full suite is green.