Constitution compliance: add native-client escape hatch

Problem

The constitution requires every provider adapter to expose an explicit native-client escape hatch. The current public MulticloudDbClient contract states that no code-level escape hatch exists, and the repository contains no nativeClient(...) implementation. Issue #38 currently lists this capability as already available, so the implementation and supportability inventory disagree.

Scope

• Design a type-safe, explicit opt-in boundary for obtaining the underlying Cosmos, DynamoDB, or Spanner client.
• Define ownership, lifecycle, logging, and unsupported-type behavior.
• Implement the contract for all three providers without changing portable default behavior.
• Correct the current-state table in #38 and update API/architecture documentation.

Acceptance criteria

• All three adapters expose their native client through one documented public escape-hatch contract.
• Incorrect requested native types fail predictably without unsafe casts.
• Native access does not weaken configuration-only portable behavior or client lifecycle safety.
• Shared contract tests and provider-specific tests cover the boundary.

Start with the public MulticloudDbClient contract and the current-state table in issue #38, then inspect the three provider adapters for their existing client and lifecycle boundaries. Define the shared escape-hatch contract and its unsupported-type behavior before implementing it for Cosmos, DynamoDB, and Spanner. Done means shared contract tests, provider-specific tests, and updated API and architecture documentation satisfy every acceptance criterion.