`run_streaming`'s `CaptureOnly` arm clones the entire child stdout into a field no caller reads

Two full copies of the child's output are made on the buffered path, one of which appears to be unused entirely.

Provenance: AI-assisted source audit, human-directed, at v0.44.2 (700bdde). Reachability was traced by reading every call site; not measured with a profiler.

1. stream.rs:475 — a clone nobody reads

In the FilterMode::CaptureOnly arm:

filtered = raw_stdout.clone();

Tracing the consumers of StreamResult on that path:

• src/core/runner.rs:107 reads .raw / .raw_stdout
• src/cmds/python/uv_cmd.rs:69 likewise

.filtered is only read on the Streaming path (runner.rs:203, search.rs:435), which is a different arm. So for every CaptureOnly invocation the process allocates a second copy of the child's stdout — up to the 10 MiB RAW_CAP — and drops it unused. 34 modules route through runner::run.

2. stream.rs:491 — a third copy, retained alongside the first two

raw = format!("{}{}", raw_stdout, raw_stderr);

StreamResult then holds raw, raw_stdout and raw_stderr simultaneously. With the 10 MiB cap applied to each stream, that is up to ~30 MiB of duplicated capture even on the capped path, against the <5 MB figure in the module docs.

3. runner.rs:14-19 — two more to_string()

let filtered = filtered.to_string();
... never_worse(raw_for_tracking, &filtered).to_string()

Both are unconditional, and never_worse already returns a &str borrow, so the second copy exists only to satisfy the return type.

Suggested directions (untested — no programmer has reviewed these)

• Leave .filtered empty (or make it Option<String>) in the CaptureOnly arm, since nothing consumes it there.
• Build raw lazily — a method that concatenates on demand, or have consumers take raw_stdout/raw_stderr directly, so all three are not resident at once.
• Return Cow<str> from emit_guarded so the common case borrows.

None of these change behaviour; they are allocation removals. Related but separate: #3392 covers rtk grep, where the same copy-everything shape produces a measured 31× memory factor.

Start by reading stream.rs around lines 475-491 and runner.rs lines 14-19, then trace the CaptureOnly consumers named in runner.rs and src/cmds/python/uv_cmd.rs. Confirm which StreamResult fields are required and preserve the existing output while removing redundant allocations. Done means behavior remains unchanged and the unnecessary copies are no longer retained.