Transfer completed Go output without copies

docs/internal/gotool.md

@@ -5,6 +5,28 @@ operation and applies the shared environment to its Go commands. It uses
 `internal/process` for cancellation, the working directory, output, and wrapped
 errors.
 
+## Command output ownership
+
+`Tool.Run` executes a status-only command. It forwards combined stdout and
+stderr to the optional writer while the child runs, then releases its capture
+on success. Failures include the captured diagnostic text and preserve the
+underlying process or forwarding error.
+
+`Tool.Output` returns combined stdout and stderr for commands whose output is
+needed by the caller, such as compiler identification. After the child stops,
+the capture's backing bytes transfer to the caller without a second copy. A
+failed output command returns both those bytes and an error containing the
+diagnostic text. The capture remains in memory for the duration of the child;
+an 8 MiB forwarded capture currently allocates about 8.39 MB per operation,
+with the returned 8 MiB slice owned by its caller. Failed output also formats
+diagnostics into the error, so the returned bytes and diagnostic string can
+coexist. Disk-backed diagnostics could reduce large failure peaks, but add file
+lifecycle and cleanup work and remain a follow-up option.
+
+Dependency preparation uses a separate command environment so proxy, private
+module, checksum, and cache settings remain available. Its completed capture
+uses the same ownership transfer when formatting a failure.
+
 ## Configuration discovery
 
 The owner asks the installed `go` command for `GOHOSTOS` and `GOHOSTARCH`, then

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Computational Efficiency Implementation Plan
 
-**Status:** Stages 1–8 complete; Stages 9–11 pending.
+**Status:** Stages 1–9 complete; Stages 10–11 pending.
 
 ## Purpose And Baseline
 
@@ -498,6 +498,24 @@ forwarded bytes and meaningful error content/cause. Run Go tool, validation,
 build, preparation, and affected CLI tests. Re-run output benchmarks and update
 [Go execution mechanics](../internal/gotool.md).
 
+**Completion:** Complete. `Tool.Run` now returns status only, while `Tool.Output`
+returns combined stdout/stderr with ownership of the completed capture buffer;
+the shared private execution path preserves settings and arguments. Status
+success drops its capture, and both status and output failures retain complete
+diagnostics and underlying process or output-writer causes. Validation and
+artifact compilation use `Run`; compiler identification uses `Output`.
+Preparation retains its network-capable environment and formats diagnostics
+from the transferred buffer. Go tool fixtures cover forwarded output, large
+failure output, cancellation, and writer failure. Go tool, validation, build,
+facade, preparation, and CLI tests passed. The combined-output benchmark now
+measures 1.0–1.1 µs and 4,176 B/2 allocations for 4 KiB, and 1.51–1.56 ms and
+8,388,699–8,388,703 B/2 allocations for 8 MiB. The Stage 2 8 MiB result was
+2.02–2.36 ms and about 16.78 MB/3 allocations, including its capture copy.
+The returned 8 MiB slice remains caller-owned in memory; failed Output also
+creates diagnostic text. Disk-backed diagnostics remain a measured follow-up
+option because avoiding their additional lifecycle and cleanup work has not
+been justified here.
+
 ## Stage 10: Cache The Runtime Metadata Fallback
 
 **Owners:** [runtimeinfo/version.go](../../internal/runtimeinfo/version.go) and