Add starts parameter to generic_bottle_cap() for multi-start threads

[HannesGitH]

Plastic bottle necks — especially the larger PET finishes (38 mm and up) — commonly use 2- or 3-start threads, but generic_bottle_cap() could only produce single-start threads. I ran into this making an adapter cap for a 38 mm PET bottle: the pitch was right, but the cap only engaged for a fraction of a turn before binding, because the neck lead is 2 * pitch.

thread_helix() already supports starts, so this just passes it through:

• New starts argument (default 1), documented in the Arguments block and the Description.
• turns is divided by starts, since thread_helix() rises pitch * starts per revolution — without that the thread would overflow the cap cavity.
• pitch keeps its existing meaning (axial distance between adjacent threads, i.e. what you measure with calipers on the neck), so the lead is pitch * starts.
• One added doc example showing a 2-start cap.

Backwards compatible: with the default starts=1 the generated geometry is unchanged — I rendered generic_bottle_cap(texture="knurled",neck_od=25,thread_od=30,height=10) before and after and the STLs are byte-identical (md5 2a525bc9150080fda6e2a76673f74858).

The 2- and 3-start variants render manifold with no warnings under --hardwarnings.

Happy to add the matching starts to generic_bottle_neck() in a follow-up (or here, if you prefer) — I kept this one minimal.