fix(pgpm): fix AST round-trip diff comparison bug and add --outputDiff flag

[pyramation]

Summary

Fixes a bug in the pgpm package command where the AST round-trip diff check was always reporting a mismatch due to comparing incompatible types. Also adds a new --outputDiff flag to export the AST diff files for debugging.

Bug fix: The original code compared tree1 (an array of RawStmt[]) to tree2 (a full ParseResult object), which would always show a diff even when the ASTs were identical. The fix now correctly compares tree1 to reparsed.stmts with the same filterStatements applied to both.

New feature: Added --outputDiff flag that writes orig.*.tree.json and parsed.*.tree.json files when a diff is detected, making it easier to debug AST mismatches.

Review & Testing Checklist for Human

• Verify the comparison fix is correct: Confirm that tree2 = filterStatements(reparsed.stmts as any, extension) properly mirrors the filtering applied to tree1 at line 62
• Test the --outputDiff flag: Run pgpm package --outputDiff on a module and verify the JSON files are written correctly when a diff exists
• Test without --outputDiff: Verify the warning message suggests using --outputDiff when a diff is detected

Recommended test plan:

1. Find or create a module that triggers an AST diff (or temporarily break the deparser)
2. Run pgpm package and verify you see the new warning message suggesting --outputDiff
3. Run pgpm package --outputDiff and verify the JSON files are written to the sql/ folder
4. Compare the two JSON files to understand the diff

Notes

• The TypeScript build has pre-existing errors due to missing @pgpmjs/* module declarations - these are unrelated to this change
• No tests were added as the user did not request them

Link to Devin run: https://app.devin.ai/sessions/369b1b00cb84494d93307674f0b9fb0e
Requested by: Dan Lynch (@pyramation)

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