releaseWizard: fix changelog forward-port for a release on an older…

[dsmiley]

… major branch

logchange.py forward-port failed releasing 9.11.0: it tried to cherry-pick 9.x feature commits onto branch_10x and main.

• Only cherry-pick commits touching changelog/v{version}/. Selecting everything under changelog/ picked up every feature commit with a changelog entry since the release branch diverged from the target.
• Pull each target branch (fast-forward only) before cherry-picking, so the final push isn't rejected.
• Resolve modify/delete conflicts confined to changelog/unreleased/ by removing the entry; -X ours doesn't settle those. Skip a pick that is already applied.
• Discard other versions' regenerated version-summary.md files, which otherwise block the next checkout.

Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com

[dsmiley]

Disclaimer: I don't do python. Just vibing with Claude here as triage issues I found. Nonetheless it had this to say when I asked for verification:

 What the real run exercised:                                                                                                                            │ 120 +    else:                                                                          
                                                                                                                                                          │ 121 +        print("  Nothing left to apply — skipping.")                               
  - Narrowed commit selection: 6 changelog-only commits per target instead of 77.                                                                         │ 122 +        git(["cherry-pick", "--skip"], cwd=git_root)                               
  - Pull before pick: all three targets fast-forwarded (they were 77, 71 and 16 behind).                                                                  │ 123 +    return True                                                                    
  - Conflict resolution: modify/delete conflicts in changelog/unreleased/ were auto-removed and the pick continued — confirmed in the log for branch_9x (4│ 124 +                                                                                   
    files).                                                                                                                                               │ 125 +                                                                                   
  - Restore after regenerate: the stray v10.0.0/version-summary.md change no longer blocked the checkout of branch_9x.                                    │ 126  def strip_unreleased_block(changelog_path: Path, dry_run=False):                   
                                                                                                                                                          │ 127      """Remove the [unreleased] block that logchangeGenerate emits.                 
  Caveats worth stating honestly:                                                                                                                         │ 128                                                                                     
                                                                                                                                                          │ 398          msg_b = f"Regenerate CHANGELOG.md for v{version}"                          
  - Two runs, not one clean pass. The first run did branch_10x and main, then failed before branch_9x. I added the restore fix and the second run         │ 399          print(f"  Committing: {msg_b!r}")                                          
    finished. The final script has not done a start-to-finish run from a clean state.                                                                     │ 400          git(["commit", "-m", msg_b], cwd=git_root, dry_run=dry_run)                
  - Skip-empty-pick path never ran. On the re-run, git reported 0 commits to pick for branch_10x and main, so that code is untested.                      │ 401 +    git(["restore", "changelog/"], cwd=git_root, dry_run=dry_run)                  
  - --push was not used. You pushed by hand, so the script's push step is untested with these changes (it wasn't modified).                               │ 402                                                                                     
  - --dry-run not run with the changes.