editor: Left-align tab whitespace marker instead of centering it

[mvanhorn]

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments (N/A)
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Tab whitespace markers (e.g. `→`) were being centered against `em_width` along with space markers. For tabs that span multiple columns (especially with proportional fonts), this pushed the arrow into the middle of the tab cell instead of aligning it where the tab actually begins, making it visually hard to tell tab from space.

Extracted the per-character centering offset into `invisible_marker_offset` and made it return zero for tabs while preserving the existing centering for spaces. Refactored `extract_whitespace_info` to use a new `invisible_paint_info` helper so the position math is shared between the renderer and the new tests.

Tests cover both branches: tabs paint at `x_for_index(start)` exactly, spaces keep their centering offset. The whitespace-map test setup is reused.

Closes #57087

Release Notes:

• Fixed - Tab whitespace markers are now left-aligned instead of centered, matching the visual position of the tab character.

AI was used for assistance.

[maxdeviant]

@mvanhorn Could you please add before and after screenshots of the change?

[mvanhorn]

@maxdeviant here's the before/after with show_whitespaces: "all" and the same demo file (mixed real-tab + space lines).

Side-by-side: https://files.catbox.moe/p9oxex.png

Before (stable main): https://files.catbox.moe/hmqoio.png
After (this PR, built from a0a7f46): https://files.catbox.moe/q2nklx.png

Demo file:

A\tB\tC\tD
\thello\tworld
\t\tdouble\ttab
    space indent

In the BEFORE shot the tab arrow sits centered inside the tab cell (you can see it offset rightward from where the tab actually starts). In the AFTER shot the arrow is at the left edge - so the tab marker visually starts where the tab character starts, instead of drifting into the middle of the cell. The space dots are unchanged.