Release 6.0.0

• chore: update dependencies (977492a)
• docs: explain stream staging costs and file upload performance (c3f28dc)
• test: cover mutation contracts and multipart payload integrity (278f763)
• fix: require an explicit file deletion acknowledgement (37ea4d2)
• fix: bound HTTP response body buffering (1066889)
• fix: support cancellation throughout upload operations (141e78d)
• fix: honor bounded Retry-After delays (216f397)
• fix: require explicit opt-in for transient request retries (1e241b7)
• chore: migrate to Oxlint (28e2c81)
• ci: update dependabot schedule (e11be50)