TASK-027 Mouse drag-select in the editor

Description

Implement in-app text selection by mouse: press to anchor, drag (MouseActionMotion) to extend, release to finish — mirroring Shift+arrow selection (respects soft-wrap via MoveToVisual). Currently handleMouse only moves the cursor on left click; there is no drag handling. Also fix the docs/help which already list mouse select as if it works (README Keys table + any in-app help).

Acceptance Criteria

• #1 Left press anchors a selection at the click position
• #2 Dragging extends the selection to the pointer (soft-wrap aware)
• #3 Release leaves the selection active; a plain click with no drag clears it and just moves the cursor
• #4 README/help mouse-select claims match actual behavior

Implementation Plan

1. RED+GREEN editor mouse-select methods: MouseAnchor(vi,col)=MoveToVisual+ClearSelection; MouseExtendTo(vi,col)=startSelection+MoveToVisual (anchor persists across motions). 2. app.handleMouse: Press->MouseAnchor; Motion->MouseExtendTo; plain click no drag leaves no selection. 3. Docs README+help.go mouse drag-select.