turboquant: ship turboquant-py + accurate bit-width errors for the library backend

Declare turboquant-py in the OCI requirements so the image (and the upgrade
delta-sync) installs it — the admin UI offered the 'library' backend but the
package was never present. The upstream 0.1.x package only accepts bit_width
1-4, so an explicitly selected library backend with turbo6/turbo8 now raises a
precise "supports 1-4 bit" error instead of the misleading "unavailable or
failed"; 'auto' still falls back to the builtin quantizer silently. Verified:
library turbo4 cos>0.994 vs originals, auto turbo8 builtin fallback intact.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EPLnsRpNBzWCHLgkXATqRz