make build-bin: on macOS, build with bun's pinned release build (closes #251)

nixpkgs points its macOS bun at an ICU in the nix store, and bun build
--compile copied that reference into bin/quak, so nix-collect-garbage could
break the cron backup. script/build-bin now downloads bun 1.3.13's release
archive on macOS too, Intel (baseline) or Apple silicon, checks its SHA-256,
and compiles with it. Those builds load only /usr/lib libraries. nix-shell is
no longer used; any system without a bun release build stops with an error.
The hash check falls back to shasum where sha256sum is missing.

Model: opus-5-5

script/build-bin

@@ -3,12 +3,13 @@
 # executable, bin/quak, which reports the version script/build stamped. Our own
 # extension to scripts-to-rule-them-all.
 #
-# bun compiles the binary from a copy of its own executable. On Linux, nix's
-# bun loads through a C library in the nix store, and so does a binary compiled
-# from it. nix-collect-garbage can delete that library, and the binary then no
-# longer starts. On Linux this script therefore compiles with bun's own release
-# build, which loads through the system's C library. It downloads that build
-# and checks it against the hash below before running it.
+# bun compiles the binary from a copy of its own executable, so the binary
+# loads the same libraries as the bun that built it. nix's bun loads libraries
+# from the nix store: on Linux its C library, on macOS its ICU. A binary
+# compiled from it loads them too. nix-collect-garbage can delete them, and the
+# binary then no longer starts. This script therefore compiles with bun's own
+# release build, which loads only the system's libraries. It downloads that
+# build and checks it against the hash below before running it.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -22,6 +23,8 @@ BUN_LINUX_X64_BASELINE_SHA256="9d8a24292a7068090205daac0a5a223f5f69736f5287e37bf
 BUN_LINUX_X64_MUSL_BASELINE_SHA256="88ca7c7ad235b498f549eea2f770f434e9f0f5e9ba95168a2d3a1f235184c394"
 BUN_LINUX_AARCH64_SHA256="70bae41b3908b0a120e1e58c5c8af30e74afae3b8d11b0d3fdd8e787ddfb4b22"
 BUN_LINUX_AARCH64_MUSL_SHA256="5385e978107ce4934298d8d6afe9bfbb898683f6cc23e6753a0da60bc60c5b81"
+BUN_DARWIN_X64_BASELINE_SHA256="a98ba6a480f22fda9b343626b906a4e26aa53618bf85d2bc5928ecf2ba45f0ed"
+BUN_DARWIN_AARCH64_SHA256="5467e3f65dba526b9fea98f0cce04efafc0c63e169733ec27b876a3ad32da190"
 
 # compile_with_downloaded_bun <archive-name> <sha256>
 compile_with_downloaded_bun() {
@@ -29,7 +32,12 @@ compile_with_downloaded_bun() {
     trap 'rm -rf "$tmp"' EXIT
     curl -fsSL -o "$tmp/$1.zip" \
         "https://github.com/oven-sh/bun/releases/download/bun-v$BUN_VERSION/$1.zip"
-    echo "$2  $tmp/$1.zip" | sha256sum -c -
+    # macOS always has shasum, but not every macOS has sha256sum.
+    if command -v sha256sum >/dev/null 2>&1; then
+        echo "$2  $tmp/$1.zip" | sha256sum -c -
+    else
+        echo "$2  $tmp/$1.zip" | shasum -a 256 -c -
+    fi
     unzip -q "$tmp/$1.zip" -d "$tmp"
     "$tmp/$1/bun" build dist/bin/quak.js --compile --outfile bin/quak
 }
@@ -59,13 +67,17 @@ main() {
             compile_with_downloaded_bun bun-linux-aarch64-musl \
                 "$BUN_LINUX_AARCH64_MUSL_SHA256"
             ;;
-        Linux-*)
-            echo "build-bin: bun has no release build for Linux on $(uname -m)" >&2
-            exit 1
+        Darwin-x86_64)
+            compile_with_downloaded_bun bun-darwin-x64-baseline \
+                "$BUN_DARWIN_X64_BASELINE_SHA256"
+            ;;
+        Darwin-arm64)
+            compile_with_downloaded_bun bun-darwin-aarch64 \
+                "$BUN_DARWIN_AARCH64_SHA256"
             ;;
         *)
-            nix-shell -p bun --run \
-                "bun build dist/bin/quak.js --compile --outfile bin/quak"
+            echo "build-bin: bun has no release build for $(uname -s) on $(uname -m)" >&2
+            exit 1
             ;;
     esac
 }