install.sh: prefer x86_64-static binary by default (portable across libc versions)

The dynamic binary requires glibc 2.38+ — built on
ubuntu-latest (2.39+), it refuses to load on Debian 12
(glibc 2.36), older Ubuntu, RHEL 8/9, etc. Hard portability
ceiling for the one-liner installer.

The musl-static binary (built on Alpine, attached as
skeletonkey-x86_64-static) runs on every libc — verified
Alpine → Debian/Ubuntu/Fedora/RHEL cross-distro. Costs ~800 KB
extra (1.2 MB vs 390 KB) but eliminates the libc-version
problem entirely.

Default: install.sh now fetches the -static asset for x86_64.
Override: SKELETONKEY_DYNAMIC=1 curl … | sh fetches the smaller
dynamic binary (for hosts that have modern glibc and want the
smaller download).

arm64: no static variant attached yet (cross-compiling musl
for aarch64 needs a separate toolchain); install.sh still
fetches the dynamic arm64 binary, which works on most modern
arm64 distros (raspberry-pi / aws graviton / etc.).

install.sh

@@ -37,7 +37,19 @@ fail()  { printf '[\033[1;31m-\033[0m] %s\n' "$*" >&2; exit 1; }
 # Detect architecture
 arch=$(uname -m)
 case "$arch" in
-    x86_64|amd64)        target=x86_64 ;;
+    # x86_64 default: the musl-static binary works on every libc
+    # (glibc 2.x of any version, musl, uclibc) — costs ~800 KB extra
+    # vs the dynamic build but eliminates the GLIBC_2.NN portability
+    # ceiling that bit users on Debian-stable / older RHEL hosts.
+    # Set SKELETONKEY_DYNAMIC=1 to fetch the smaller dynamic build
+    # (needs glibc >= 2.38, i.e. Ubuntu 24.04 / Debian 13 / RHEL 10).
+    x86_64|amd64)
+        if [ "${SKELETONKEY_DYNAMIC:-0}" = "1" ]; then
+            target=x86_64
+        else
+            target=x86_64-static
+        fi
+        ;;
     aarch64|arm64)       target=arm64  ;;
     *) fail "Unsupported architecture: $arch (only x86_64 and arm64 currently)" ;;
 esac