Problem: pressing `o`/`O` in a canola buffer placed the cursor at
column 0, requiring manual navigation past concealed ID prefixes and
column text (icons, permissions) to reach the name column.
Solution: add `show_insert_guide()` which temporarily sets
`virtualedit=all` on empty lines and positions the cursor at the
name column. Computes the correct virtual column by measuring the
visible column prefix width via `nvim_strwidth`, adjusting for
`conceallevel` (0=full ID width, 1=replacement char, 2/3=hidden).
Restores `virtualedit` on `TextChangedI` or `InsertLeave`.
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