Closes #11140.

Console selection previously persisted screen-space mouse coordinates after releasing the mouse. This made a completed selection unstable when backlog lines were appended, the console was scrolled, or the layout changed.

Selection endpoints now use stable backlog-entry IDs and glyph offsets. Rendering and clipboard extraction resolve the selected content from these anchors, so a completed selection remains attached to the same text across appends, scrolling, and layout changes.

Also addresses the related selection instability described in #12265.

Checklist

• Tested the change ingame
• Provided screenshots if it is a visual change
• Tested in combination with possibly related configuration options
• Written a unit test (especially base/) or added coverage to integration test
• Considered possible null pointers and out of bounds array indexing
• Changed no physics that affect existing maps
• Tested the change with ASan+UBSan or Valgrind's memcheck (optional)
• I didn't use generative AI to generate more than single-line completions

AI assistance: AI was used to explore the existing console selection implementation and draft parts of the change. I reviewed the design and diff, built the client, ran the test suite, and manually tested the behavior in-game.