docs(pre2): add VM image test guide

[sqlrush]

Summary

• Add docs/pre2/vm-test-guide.md, a step-by-step test guide for the v0.135.0 ARM64 single-VM image. It covers:
  • architecture and how local PGDATA relates to the shared data directory;
  • cluster features: Cache Fusion 2-way and 3-way transfer, PCM block states and past images, GRD, GES, LMD deadlock detection, global SCN, undo/TT/ITL multi-versioning, per-instance WAL threads, membership and quorum, and background processes;
  • download, verification, installation, connection, hands-on labs, normal stop and restart, recovery steps, test recommendations and how to collect logs.
• Add the companion lab scripts under docs/pre2/sql/.
• Link the guide from docs/pre2/README.md and docs/pre2/vm-image.md.
• The guide has 12 Mermaid diagrams and uses GitHub alert blocks.

Test plan

• Downloaded the v0.135.0 release assets with the documented curl loop and ran the verification, merge and extract commands; every check printed OK.
• Imported fresh copies of the image and ran each lab in document order: status checks, cross-node read/write, Cache Fusion counter deltas, row-lock wait (GesTxEnqueueWait), table-lock conflict (55P03), consistent read, pgbench with 4 clients per instance (no failed transactions; the four nodes read the same totals; the total increase equals the number of committed transactions), deadlock detection (40P01), SCN, WAL threads and shared files.
• Ran ./stop.sh, ./start.sh and ./test.sh again after the restart; data was preserved.
• Parsed all Mermaid blocks with mermaid 11; checked that every table-of-contents anchor exists on the rendered branch page.
• After merge, confirm that the raw script URLs on main resolve.