Shifter: Chain Utils: Add locator removal and fix single locator add offset

[zebraed]

Description of Changes

Extends Chain Utils with a way to shorten chains and fixes an existing
Add Locator bugfix.

• Remove Locator from Chain: Removes the last locator of each selected
  chain. Only the tip is removed, child component guides are reparented to
  the new chain end, the display curve is regenerated, and at least one
  locator is always preserved.
• Fix Add Locator on single-locator chains: When a chain had only one
  locator, the new locator was created at the same position as the existing
  one. The component root is now used as the direction source so the new
  locator is placed correctly.
• One undo: Both operations are wrapped so each runs as one undo step.
• Display curve: Fix missing root-to-first-locator curve: Regeneration used locators only, so the root-to-first-locator segment was not drawn. Now includes the root.
• Updated the Chain Utils user documentation.

Testing Done

Maya2027 / Windows

• Removed the tip locator and confirmed child guides were reparented and the
  display curve updated.
• Added a locator to a multi-locator chain.
• Added a locator to a single-locator chain and confirmed correct placement.
• Confirmed each add/remove reverts with a single undo.

Click to view the testing video

testing_for_remove_add_locators.mp4

Related Issue(s)

N/A

Thank you.