Add piped child process support

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Computational Efficiency Implementation Plan
 
-**Status:** Stages 1–2 complete; Stages 3–11 pending.
+**Status:** Stages 1–3 complete; Stages 4–11 pending.
 
 ## Purpose And Baseline
 
@@ -200,6 +200,13 @@ credentials and mutable external services.
 
 **Owner:** [internal/process](../../internal/process/process.go) and its tests.
 
+**Completion:** Complete. `Run` and `StartPiped` share command construction.
+Piped children expose caller-owned input and output pipes, cancellation, and a
+single underlying wait/reap while preserving parent-context and exit errors.
+Tests cover argument, directory, and environment propagation; an exchange that
+responds before input EOF; synchronized cancellation; startup failure; and a
+nonzero exit. The package inventory now describes the pipe lifecycle.
+
 1. Factor existing command construction and command-error wrapping into small
    private helpers shared by `Run` and a new internal `StartPiped` operation.
    Preserve arguments, directory, environment overlay/removal, stderr routing,