Per-turn context window and what fills it

[fylorn]

What

A session's turns only carried their measured usage. The desktop app wants to show how full a turn's context window was and what was occupying it.

• TurnView gains context_window: number | null: the window of the model that answered the turn, resolved now through the one spec resolution core already has (tw_config::model_specs: the upstream's manual model_specs first, then the price table), looked up by the model name the upstream actually received (sent_model, now carried by TurnRow).
• New endpoint GET /sessions/{id}/turns/{turn}/context answering TurnContext:
  • parts: the stored request body decoded with the client's dialect (Anthropic system + messages; Chat system/developer messages; Responses instructions + input items; Gemini systemInstruction + contents) and estimated in one pass as system, tools (tool definitions), history (every message before the last user message), last_user (the last user message with the tool results it carries) and total, by the same estimator count_tokens answers with (tw_engine::estimate_parts, the per-part twin of estimate_tokens; each part rounds on its own, so the sum can differ from the whole-body estimate by a few tokens).
  • window, input_tokens, cache_read_tokens from the spec and the history row.
  • kept: false with parts: null when the body was never kept, has expired, was truncated or cannot be decoded.
  • 404 control.session_not_found for an unknown session; 404 control.request_not_found for a turn that is not in that session.
• CONTROL_API_VERSION 44 → 45, with the version notes above it; the new types are in the TypeScript export (not kept in-tree; CI regenerates it).
• tw-control now depends on tw-dialect directly to decode stored bodies (one line in Cargo.lock).

New messages

None. The endpoint reuses control.session_not_found and control.request_not_found.

Verified

• cargo fmt --all --check
• cargo clippy -p tw-api -p tw-engine -p tw-store -p tw-control -p twcore --all-targets -- -D warnings
• cargo test --workspace with the local proxy unset: all green
• cargo run -p tw-api --features ts --example export_ts: exports TurnContext, ContextParts, TurnView.context_window
• New tests: crates/tw-control/tests/turn_context.rs (Anthropic, Chat, Responses and Gemini bodies split into four parts; body gone; manual spec vs price table vs unknown model; the two 404s), tw_engine::facts::tests::parts_split_the_body_at_the_last_user_message, and a ts.rs test pinning the exported shape.

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