Add Insight SQLite diagnostic store

[josiahking]

Summary

• add a deterministic versioned diagnostic snapshot codec
• add an optional SQLite diagnostic store using a caller-supplied PDO connection
• preserve exact execution identifiers, observation order and nullable snapshot values
• reject duplicates and corrupt or incompatible persisted diagnostic payloads
• keep SQLite storage explicit and separate from application database configuration

Validation

• focused codec and SQLite storage tests passed
• full Insight test suite passed with SQLite enabled
• root PHPUnit, PHPStan, dependency architecture and style checks passed
• strict Composer, supply-chain, package split, prerelease consumer and application skeleton validation passed