Fix CJK lines wrapping in the overlay (exact wide-char padding)

[cocomode18]

Problem

In panes containing CJK text, every line wraps in the fingers overlay, leaving a
blank row beneath it — the visible content appears to grow / become double-spaced
the moment fingers is triggered. ASCII-only lines are unaffected.

Root cause

Hinter#process_line pads each rendered line to the pane width, correcting for
double-width characters with:

double_width_correction = ((line.bytesize - line.size) / 3).round.to_i

This /3 is tuned for 4-byte emoji (3 extra bytes → +1 column) but under-counts
3-byte CJK characters: a CJK char has bytesize - size == 2, so it contributes
2/3 ≈ 0.67 instead of the 1 extra column it actually occupies.

The correction therefore comes out too small, padding_amount too large, and the
rendered line exceeds the pane width by ~N/3 columns (N = CJK chars on the line).
The terminal wraps it, and the trailing "\n" then lands a row lower — so each
CJK line consumes two rows. Worked example at width 100:

line	wide chars	rendered display width
最終形:	3	101 → wraps
- リンク開く → Shift+Cmd+Click …	16	105 → wraps
* Crunched for 5s (ASCII)	0	100 → exact

Fix

Count wide characters exactly instead of estimating from byte length, using the
Unicode East Asian Width Wide/Fullwidth ranges plus the common emoji blocks.
Emoji remain wide, so they are unaffected; ambiguous-width symbols (box-drawing,
powerline glyphs) are deliberately excluded because terminals render them one
column wide.

Tests

Adds specs covering CJK line padding (no overflow at the target width), exact
wide-char counting, and emoji non-regression. crystal spec spec/lib/fingers/hinter_spec.cr
passes (6 examples, 0 failures).