Align serialization contract docs with schema version 6

[ExplodingUFO]

Context

GraphDocumentCompatibility.CurrentSchemaVersion is currently 6, and SerializationCompatibilityTests already assert that GraphDocumentSerializer writes "SchemaVersion": 6. The EN/ZH serialization contract docs and SerializationContractDocsTests still describe the current workspace/document schema as 5, which can mislead hosts reading the persistence contract.

Scope

• Update EN/ZH serialization contract docs so the current workspace/document schema version matches GraphDocumentCompatibility.CurrentSchemaVersion = 6.
• Update focused docs tests to defend the schema 6 wording and the accepted legacy/current range.
• Keep the change docs/tests-only.

Out of scope

No serializer behavior change, no GraphDocument schema change, no schema version bump, no migration behavior change, no workspace save/load integration change, no clipboard/export behavior change, no UI work, and no React Flow parity claim expansion.

Acceptance Criteria

• docs/en/serialization-contracts.md states the current workspace/document schema version is 6 and accepts 1 through 6 where applicable.
• docs/zh-CN/serialization-contracts.md states SchemaVersion = 6 and accepts 1 through 6 where applicable.
• SerializationContractDocsTests fails on stale schema 5 wording and passes with schema 6 wording.
• Verification includes the focused docs test and git diff --check.