fix(checkpoint): ignore out-of-workspace rewind snapshots

[GTC2080]

Closes #4246

Problem

Code-only rewind can fail on Windows with an error like rewind code: checkpoint path ... escapes workspace ... when a checkpoint contains a path outside the active project root. In that state the desktop shows the rewind action, but the restore aborts before the user's project files are rolled back.

Root cause

Checkpoint snapshots recorded writer-preview paths verbatim. In project tabs, that allowed absolute paths outside WorkspaceRoot (for example Reasonix-managed project or memory files) to be persisted beside normal workspace edits. RestoreCode later rejected those paths with the workspace escape guard, which is safe but made one out-of-scope snapshot fail the whole code rewind. Existing sessions could keep hitting the same failure because the bad path was already persisted.

Fix

• Normalize in-workspace snapshot paths to workspace-relative slash paths before storing them.
• Skip new snapshots whose preview path is outside the checkpoint workspace root.
• Keep the restore-time escape guard, but skip legacy out-of-workspace checkpoint entries instead of failing the entire restore.
• Switch the root containment check to filepath.Rel, avoiding fragile string-prefix comparisons.

Validation

• go test ./internal/checkpoint -run 'TestRestoreSkipsPersistedPathEscape|TestSnapshotSkipsOutsideWorkspaceWithoutBreakingRestore|TestSnapshotStoresWorkspaceRelativePaths' -count=1
• Clean detached worktree at this commit:
  • gofmt -l .
  • git diff --check HEAD^ HEAD
  • go test ./...
  • go test ./... in desktop/
  • go vet ./...
  • go vet ./... in desktop/
  • go build ./...
  • go build ./... in desktop/
  • npm run test:all in desktop/frontend
  • npm run build in desktop/frontend