build: scripts/sccache.sh wraps rustc when sccache is installed, and the gates source it

A build cache serves a dependency rebuild after a branch switch or a `git clean` instead of
recompiling it, which is most of what the in-loop verify spends. It stays a convenience and
never a requirement: with nothing on `PATH` nothing is exported and a clone needs only stable
Rust and a C compiler, exactly as before.

`check.sh`, `gates.sh` and `smoke.sh` source it, so the loops pick it up without being asked
and a machine that would rather not have it sets `BENILLA_NO_SCCACHE=1`. An existing
`RUSTC_WRAPPER` wins. Only rustc is wrapped — the vendored Lua build wants its own
`CC="sccache cc"`, so it is not cached by this.

scripts/check.sh

@@ -15,6 +15,9 @@ root="$(git rev-parse --show-toplevel 2>/dev/null || true)"
 [ -n "$root" ] && [ -f "$root/scripts/check.sh" ] || root="$here"
 cd "$root" || exit 1
 
+# sccache, when installed, caches the builds below; opt out with BENILLA_NO_SCCACHE=1.
+. "$root/scripts/sccache.sh"
+
 if [ "${CHECK_FULL:-}" = "1" ]; then
     exec "$root/scripts/gates.sh"
 fi

scripts/gates.sh

@@ -18,6 +18,9 @@ here="$(cd "$(dirname "$0")/.." && pwd)"
 root="$(git rev-parse --show-toplevel 2>/dev/null || true)"
 [ -n "$root" ] && [ -f "$root/scripts/gates.sh" ] || root="$here"
 cd "$root" || exit 1
+
+# sccache, when installed, caches the builds below; opt out with BENILLA_NO_SCCACHE=1.
+. "$root/scripts/sccache.sh"
 echo "gating: $root"
 
 # ── Green-stamp memoization ──────────────────────────────────────────────────────────────────────

scripts/sccache.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+# Use sccache for rustc when it is installed. It is a developer convenience, never a requirement:
+# without it nothing is exported and a clone needs only stable Rust and a C compiler.
+#
+# Sourced, it exports RUSTC_WRAPPER into the current shell for later cargo commands; run with a
+# command, it wraps just that command. The verify, land and smoke scripts source it themselves.
+#
+#   . scripts/sccache.sh && cargo play
+#   scripts/sccache.sh cargo play
+#
+# An existing RUSTC_WRAPPER wins, BENILLA_NO_SCCACHE=1 skips this, and SCCACHE_DISABLE=1 turns
+# sccache itself off. Only rustc is wrapped: the vendored Lua build wants its own `CC="sccache cc"`.
+# Sourced, it must not change the caller's shell options.
+enable() {
+    [ -n "${RUSTC_WRAPPER:-}" ] && return 0
+    [ -n "${BENILLA_NO_SCCACHE:-}" ] && return 0
+    command -v sccache >/dev/null 2>&1 || return 0
+    export RUSTC_WRAPPER=sccache
+}
+
+enable
+# Executed with a command: run it wrapped. Sourced: leave the caller's positionals alone.
+if [ "${BASH_SOURCE[0]}" = "$0" ] && [ "$#" -gt 0 ]; then
+    exec "$@"
+fi

scripts/smoke.sh

@@ -16,6 +16,9 @@ if [ -z "$root" ] || [ ! -f "$root/scripts/smoke.sh" ]; then
     root="$(cd "$(dirname "$0")/.." && pwd)"
 fi
 cd "$root" || exit 1
+
+# sccache, when installed, caches the client build below; opt out with BENILLA_NO_SCCACHE=1.
+. "$root/scripts/sccache.sh"
 echo "smoke: $root"
 
 # Resolve the account first, then unset every inherited WOW_* the case below does not keep, so