Rust perf: stop cloning whole turn streams in analyze hot paths

[willwashburn]

Context

Multiple analyze-module hot paths clone the full input Vec<TurnRecord> before they aggregate, doubling the working-set memory of the most expensive verbs. Every TurnRecord carries a Vec<ToolCall> plus optional file lists — these are not cheap clones.

Concrete sites:

• crates/relayburn-sdk/src/analyze/overhead.rs:131-136 — clones every applicable TurnRecord per overhead file, 3×. With three default candidates and overlapping applies_to, every Claude turn ends up cloned ~2× and every Codex/OpenCode turn ~1×. Biggest unforced alloc in the analyzer.
• crates/relayburn-sdk/src/analyze/hotspots.rs:208-216 — the entire turn stream cloned into IndexMap<String, Vec<TurnRecord>> via entry(t.session_id.clone()).or_default().push(t.clone()).
• crates/relayburn-sdk/src/analyze/quality.rs:113-124 — parallel Vec<(String, Vec<TurnRecord>)> + HashMap<String, usize> index, cloning every turn.
• crates/relayburn-sdk/src/analyze/compare.rs:127-152 — model id cloned 4× per turn inside the hot accumulation loop (lines 132, 140, 143, 147).

Proposed fix

Switch the per-session/per-file aggregations to Vec<&TurnRecord> (or IndexMap<String, Vec<&TurnRecord>>). Nothing downstream of these sites mutates the turns; they only read them. The lifetime should already trivially be the input slice.

For compare.rs:127-152 specifically: hoist the model-name clone above the entry/or_insert_with calls (one clone per new model, not per turn).

Also fold in:

• crates/relayburn-sdk/src/analyze/overhead.rs:170-177 describe_applies_to — three serde_json::to_value round-trips on every call to discover that SourceKind::ClaudeCode serializes to \"claude-code\". A 4-arm match returning &'static str removes 3 allocs per file render. (Belongs with the enum-string-conversion follow-up too.)

Verification

The conformance gate (deep-equal results vs TS @relayburn/sdk) is the existing safety net; behavior must not change.

References

• Analyze review notes from the May 2026 Rust review.