v0.12.1

Builds under the repo's own manifest.scm (alsa-lib was already listed; the build simply was not run under it). Adds dev-dependencies so the native test harness can be built. Scopes -fno-sanitize=undefined to the two VENDORED compile steps, so libvorbis's ((lo-1)<<16) at psy.c:319 no longer traps at runtime and kills the process mid-suite.

Stops tracking .sigil/deps/sigil-stdlib, a symlink holding an absolute path into one machine's dependency store.

Released to unblock sigil-studio, which cannot compile the published tip: it predates the with-sigil-c-flags fix and fails on a missing sigil-internal.h.