[osipovartem]

Summary

Add native Snowflake ALTER SCHEMA ... SET and UNSET property syntax behind a default-off dialect capability. Keep the existing parenthesized option syntax unchanged.

• Preserve ordered assignments in typed AST variants, including duplicate options for semantic validation by callers.
• Accept documented whitespace, comma and newline assignment separators; keep unset properties comma-separated.
• Cover display/reparse, spans, serde/visitors, quoted names, malformed boundaries and other dialects.

Snowflake ALTER SCHEMA reference

Verification

• All-feature parser suite: 1,610 passed, 0 failed, 7 ignored.
• Focused default-feature tests: 9 passed.
• Strict all-target/all-feature Clippy and formatting passed.
• Independent read-only review: approved with no blocking findings.

This is syntax support, not a claim that every property is implemented by Rustice. The companion runtime change admits only literal COMMENT set/unset.