build(Makefile): add per-target timing via SHELL wrapper (#22862)

pre-commit and pre-push only reported total elapsed time at the end,
making it hard to identify which jobs are slow.

Add a `MAKE_TIMED=1` mode that replaces `SHELL` with a wrapper
(`scripts/lib/timed-shell.sh`) to print wall-clock time for each
recipe. pre-commit and pre-push enable this on their sub-makes.

Ad-hoc use: `make MAKE_TIMED=1 test`

scripts/lib/timed-shell.sh

@@ -0,0 +1,41 @@
+#!/usr/bin/env bash
+# timed-shell.sh wraps bash with per-target wall-clock timing.
+#
+# Recipe invocation:    timed-shell.sh <target> -ceu <recipe>
+# $(shell ...) calls:   timed-shell.sh -c <command>
+#
+# Enable via Makefile:
+#   SHELL := $(CURDIR)/scripts/lib/timed-shell.sh
+#   .SHELLFLAGS = $@ -ceu
+#
+# $(shell ...) uses SHELL but passes -c directly, not .SHELLFLAGS.
+# Detect this and delegate to bash without timing output.
+if [[ $1 == -* ]]; then
+	exec bash "$@"
+fi
+
+set -eu
+
+target=$1
+shift
+
+bold=$(tput bold 2>/dev/null) || true
+green=$(tput setaf 2 2>/dev/null) || true
+red=$(tput setaf 1 2>/dev/null) || true
+reset=$(tput sgr0 2>/dev/null) || true
+
+start=$(date +%s)
+echo "${bold}==> ${target}${reset}"
+
+set +e
+bash "$@"
+rc=$?
+set -e
+
+elapsed=$(($(date +%s) - start))
+if ((rc == 0)); then
+	echo "${bold}${green}==> ${target} completed in ${elapsed}s${reset}"
+else
+	echo "${bold}${red}==> ${target} FAILED after ${elapsed}s${reset}" >&2
+	exit $rc
+fi