build(steam): the SDK zip IS the only download — ContentBuilder included, links recorded

The partner dashboard offers exactly one download because there is only one: the SDK zip
carries the runtime (steam_api64.dll) and the release tooling (sdk/tools/ContentBuilder
with steamcmd.exe for Windows and steamcmd.sh for Linux/macOS, plus the app_build and
depot_build VDF templates). Verified against the zip rather than assumed.

scripts/steam-sdk.sh gains --contentbuilder, opt-in because it is 27 MB of binaries that
only the uploading machine needs.

Distribution.md §2 now opens with that fact — there is no separate ContentBuilder
download to hunt for — plus a reference table of the pages that actually get used:
SteamPipe upload, DLC setup, BIsDlcInstalled, store artwork sizes, Coming Soon window,
the Wallet rule, recurring subs, partner support and Steamworks.NET.

scripts/steam-sdk.sh

@@ -7,6 +7,11 @@
 # is only needed on the machine that uploads a depot.
 #
 #   ./scripts/steam-sdk.sh ~/Downloads/steamworks_sdk_165.zip
+#   ./scripts/steam-sdk.sh ~/Downloads/steamworks_sdk_165.zip --contentbuilder
+#
+# --contentbuilder also extracts sdk/tools/ContentBuilder into steam/contentbuilder/ — that is
+# the release tooling (steamcmd + the app_build/depot vdf templates). It is a separate opt-in
+# because it is only needed on the machine that uploads a depot, and it is ~12 MB of binaries.
 #
 # steam_api64.dll is Valve's redistributable — shipping it beside a Steam-published exe is
 # permitted by the Steamworks terms. It is NOT committed: steam/ is gitignored, like ffmpeg/.
@@ -14,6 +19,7 @@
 set -euo pipefail
 
 ZIP="${1:-$HOME/Downloads/steamworks_sdk_165.zip}"
+WANT_CONTENTBUILDER="${2:-}"
 ROOT="$(cd "$(dirname "$0")/.." && pwd)"
 
 [ -f "$ZIP" ] || { echo "❌ SDK zip not found: $ZIP" >&2; echo "   Download it from partner.steamgames.com/downloads/steamworks_sdk.zip while signed in." >&2; exit 1; }
@@ -41,5 +47,29 @@ PY
 printf '480\n' > "$ROOT/steam/steam_appid.txt"
 echo "✓ steam/steam_appid.txt = 480 (Spacewar, local Debug testing only)"
 
+if [ "$WANT_CONTENTBUILDER" = "--contentbuilder" ]; then
+  python3 - "$ZIP" "$ROOT" <<'PY'
+import sys, zipfile, pathlib
+
+zip_path, root = sys.argv[1], pathlib.Path(sys.argv[2])
+prefix = "sdk/tools/ContentBuilder/"
+dest = root / "steam" / "contentbuilder"
+
+count = 0
+with zipfile.ZipFile(zip_path) as z:
+    for name in z.namelist():
+        if not name.startswith(prefix) or name.endswith("/"):
+            continue
+        target = dest / name[len(prefix):]
+        target.parent.mkdir(parents=True, exist_ok=True)
+        with z.open(name) as src, open(target, "wb") as out:
+            out.write(src.read())
+        count += 1
+
+print(f"✓ steam/contentbuilder/ ({count} files)")
+print("  Windows upload: steam/contentbuilder/builder/steamcmd.exe +login <user> +run_app_build <vdf>")
+PY
+fi
+
 echo ""
 echo "Next: the entitlement seam (IEntitlement + SteamApps.BIsDlcInstalled)."