v1.0: issues sweep, milestone, and CONTRIBUTING

[espg]

🤖 from Claude

Sub-issue of #48 (v1.0 sweep), release-org (last).

Scope

The organizational pieces espg listed for the release:

• Issues sweep: triage open issues against the v1.0 contract — label what blocks 1.0, defer the rest to 1.x.
• Milestone: create a 1.0 milestone and attach the blocking sub-issues of Version 1.0 Sweep (Follow up to new morton datatype) #48.
• CONTRIBUTING: a short contributor guide (build via maturin per BUILDING.md, the claude/* branch + draft-PR workflow, the test/lint gates).

Acceptance criteria

Milestone created and populated; open issues triaged; CONTRIBUTING.md added. Milestone creation / cross-issue relabeling are side-effecting — done only on explicit @espg go-ahead.

Refs #48.