docs(resume): document cross-version checkpoint compatibility

[Kiran01bm]

What and Why ?

Adds a "Cross-version compatibility" section to pkg/migration/resume-from-checkpoint.md explaining that checkpoint tables are version-specific by design.

Spirit deliberately uses SELECT * when reading the checkpoint table so that any schema drift (columns added, removed, or reordered in a newer release) fails the read. This is intentional — silently reading a checkpoint with a different schema risks misinterpreting fields and corrupting the migration. The behavior was previously documented only in a Go comment in runner.go (resumeFromCheckpoint) and was not surfaced in the user-facing markdown.

What the new section covers

• Why SELECT * is intentional and what it guards against.
• Upgrade mid-migration (older binary → newer binary) and rollback mid-migration (newer → older) both fail the read.
• Non-strict mode falls through to newMigration() and restarts the copy from scratch silently.
• Strict mode currently surfaces this as a generic "could not read from table" error rather than a typed status.Err… value (called out as a known observability gap).
• Operational guidance:
  • Finish in-flight migrations on the same Spirit version they were started with.

Scope

Docs-only change. No code or test changes.

[chatgpt-codex-connector]

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