userland: search and exec tools, spawn library (T14)

rg (ripgrep 15.2), grep (ours, on ripgrep's regex crates), find and xargs
(ours), sed (uutils 0.3 wrapper), diff and cmp (uutils diffutils 0.6 with -r),
fd 10.5, jaq 2.3, env and timeout. userland/spawn is a small Command-like
client of the process interface. search-test.nu: 42 cases match native tools.

userland/diffutils/PATCHES.md

@@ -0,0 +1,14 @@
+# Recursive directory comparison
+
+Source: crates.io `diffutils` 0.6.0, https://github.com/uutils/diffutils.
+The vendor copy retains the upstream licenses.
+
+The public library exposes comparison functions but not the diff CLI entry point.
+The vendor copy retains that CLI parser and dispatcher.
+
+- `src/params.rs` accepts `-r` and `--recursive`.
+- `src/diff.rs` recursively compares directory entries in lexical order.
+- The recursive path reuses the upstream file comparison implementation.
+- `tools.txt` exposes the component as `diff` and `cmp`.
+
+`search-test.nu` compares identical directories and changed nested files against GNU diff.