build: D1 cross-compile via cargo-zigbuild (4 non-macOS targets)

Replace the single-target musl cc with cargo-zigbuild + zig in the flake
devShell — one universal cross cc/linker (incl. rusqlite's bundled SQLite
C) for all four non-macOS D1 targets, added to rust-toolchain.toml:
  x86_64/aarch64-unknown-linux-musl              (static, D2)
  x86_64-pc-windows-gnu, aarch64-pc-windows-gnullvm  (standalone .exe)

Windows links -lsynchronization (std WaitOnAddress), which rust-overlay's
toolchain and zig's mingw don't ship; the symbols are forwarded by
kernel32, so an empty stub libsynchronization.a (ci/winstub/, wired via
.cargo/config.toml for the windows targets only) satisfies the linker.
Verified: all four build; linux static; windows valid PE32+.

ci/winstub/libsynchronization.a

@@ -0,0 +1 @@
+!<arch>