feat: Handle all forms of range requests

[gschulze]

Closes #259.

Adds support for all forms of range requests documented by fsspec.

Implemented:

• get_range/get_ranges cover all three GetRange variants: bounded, start alone (to the end of the object), and negative start (the last N bytes). get_ranges takes None per element in ends/lengths, and resolves open-ended ranges with one head request so they still coalesce.
• cat_file accepts either bound alone, or either counting back from the end. cat_ranges accepts fsspec's full vocabulary and now honors max_gap, which was previously accepted and silently ignored. test_cat_ranges_mixed is no longer xfail.

API changes:

• Python:
  • ends/lengths: Sequence[int] | None → Sequence[int | None] | None.
  • coalesce: int → int | None, where None means obstore's own default, previously hardcoded in four places across _get.pyi and store.py.
  • start stays int, but negative values are now accepted.
• Rust:
  • start/starts: u64 → i64 on all four get_range*/get_ranges*, to support suffix requests.
  • ends/lengths: Option<Vec<u64>> → Option<Vec<Option<u64>>>.
  • coalesce: u64 → Option<u64>.

Behavior changes:

get_ranges(starts=[5]) reads to the end of the object instead of raising; ends and lengths may both be passed if at most one is non-None per range; mismatched sequence lengths raise ValueError instead of being silently truncated.

Follow-up tasks:

Raise upstream: obstore now accepts range forms obspec's GetRange/GetRanges docstrings declare invalid. Conformance is unaffected, but code using the new forms won't port to other obspec implementations.