docs: add physical storage UUID RFC

[bb7133]

What problem does this PR solve?

Issue Number: ref #69923

Problem Summary:

This PR adds an RFC for introducing a globally restorable physical storage identity for TiDB tables and partitions. The goal is to enable supported BR restore and TiDB-X tenant migration flows to preserve physical data key prefixes instead of rewriting source physical table IDs to target-local physical table IDs.

What changed and how does it work?

This PR adds a new design document under docs/design/:

• docs/design/2026-07-19-physical-storage-uuid.md

The RFC describes:

• why logical UUID metadata alone cannot remove BR table ID key rewrite;
• the proposed PhysicalStorageUUID metadata for tables and partitions;
• UUIDv7 generation semantics for new physical data spaces;
• a possible UUID-based tablecodec v2 key prefix;
• DDL, BR restore, and TiDB-X tenant migration semantics;
• rollout, testing, compatibility, and open questions.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

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