Introduce PlanCacheSchemaKey and multi-schema QueryCacheKey (cross-schema 1/6)

[arnaud-lacurie]

Summary

• Introduces PlanCacheSchemaKey — a stable key component derived from a set of (schemaName, schemaVersion) pairs — so that a cached plan is invalidated when any participating secondary schema changes
• Extends QueryCacheKey to carry the secondary-schema set alongside the primary schema, giving each multi-schema query its own cache slot
• No behaviour change for single-schema queries; the secondary set is empty and the key is identical to before

Stacking

This is PR 1 of 6 in the cross-schema join series. All PRs target main; rebase each one after the previous merges.

#	PR	Topic
1	this	Plan-cache key for multi-schema queries
2		SchemaIdentifier tag + store-binding infrastructure
3		Secondary-store wiring into execution context
4		Type-namespace collision avoidance + catalog metadata
5		Java integration tests
6		YAML integration tests + documentation

Test plan

• Plan cache unit tests pass
• Multi-schema queries get distinct cache entries from single-schema queries
• No regression in single-schema plan-cache hit rate