[osipovartem]

Summary

Parse Snowflake ALTER DATABASE [IF EXISTS] name SET key = <primitive literal> and UNSET key, ... into native AST, with Display, spans, serde and visitor support.

• Keep the capability off by default; Snowflake enables it, and custom dialects can opt in independently of ALTER SCHEMA.
• Share the existing primitive namespace-property parser without broadening ALTER SCHEMA semantics. Preserve ordered/duplicate properties for downstream validation.
• Reject complex properties and malformed separators; preserve literal payloads and statement boundaries. No runtime support is implied for every parsed property.
• Add eight focused tests, including the immutable Rustice Snowflake COMMENT examples, dialect boundaries, serialization and visitors.

Syntax reference: Snowflake ALTER DATABASE.

Validation

• All-feature native/integration tests: 1,571 passed, zero failed, seven ignored.
• All-feature doctests: 47 passed. Combined: 1,618 passed.
• All-target/all-feature Clippy with -D warnings, formatting and whitespace checks pass.
• Independent read-only review: approved, no blocking findings.

This is a bounded parser change for the Oracle P0 database-comment workflow; runtime ownership, RBAC and persistence qualification are separate.