Validated against a real thread: request_token_usage is keyed by user
message and covered only part of the requests (cumulative was ~3x the
map sum), so per-request calls are now supplemented by one remainder
entry that brings each thread exactly to cumulative_token_usage. Live
run matches the store token-for-token on all four fields.
Zed's DataType enum carries both zstd (the current save path) and json;
verified against crates/agent/src/db.rs, along with the exact TokenUsage
field names, the SerializedLanguageModel {provider, model} shape, and
RFC3339 updated_at serialization.
Reads Zed's agent threads from the single threads.db SQLite store: each
row's zstd-compressed JSON carries the thread model and per-request
Anthropic-shaped token usage, so calls are emitted per request with
exact input/output/cache fields and priced through the existing engine.
In-progress threads with an empty per-request map fall back to the
cumulative counter. zstd comes from node:zlib, which ships it from
22.15; on older Nodes the provider skips with a notice instead of
failing. On-disk format documented by @chatzinikolakisk in #480 and the
schema verified against a real Zed install.
