yoda: snap monospace cell width via Unicode East Asian Width

Replaces the previous heuristic (font_monospace_em_width.is_none() ⇒ 2
cells) which was reviewed as unsound: Arabic, dingbats, math symbols and
other narrow scripts pulled from non-monospace fallback fonts would all
have been forced to 2 cells. It also didn't handle ZWJ emoji clusters
or ambiguous-width chars correctly.

Proper fix, computed at shape time when `line: &str` is in scope:
- new ShapeGlyph.terminal_cells: u8 (0, 1 or 2)
- populated via unicode-width crate applied to the cluster text
  line[start..end] (harfrust path, uses UnicodeWidthStr) or to the
  single codepoint (no-font fallback path, UnicodeWidthChar)
- layout_to_buffer consumes it when match_mono_width is Some:
      x_advance = cells * mono_width
  instead of the previous round(x_advance / mono_width) * mono_width
  which produced variable cell counts for fallback glyphs.

Covers:
- ASCII + Latin        → width 1  (unchanged visual)
- CJK + fullwidth      → width 2  ✓
- Emoji (incl. ZWJ)    → width 2  ✓  (cluster text handles the ZWJ case)
- Arabic / Hebrew      → width 1  ✓  (was wrongly snapped to 2 before)
- Combining marks      → width 0  ✓  (zero-advance, matches terminals)
- Variation selectors  → width 0  ✓

Limitations: ambiguous-width chars (EAW=A) resolve to 1 via unicode-width
default; a 'cjk' ambiguous mode (unicode-width::UnicodeWidthChar::width_cjk)
could be exposed later as a Buffer flag if needed — not needed for typical
terminal use, matching most wcwidth implementations.

Based on review feedback from lionel@wopr.io on the initial heuristic patch.

Cargo.toml

@@ -30,6 +30,8 @@ sys-locale = { version = "0.3.2", optional = true }
 unicode-linebreak = "0.1.5"
 unicode-script = "0.5.8"
 unicode-segmentation = "1.12.0"
+# Yoda: EAW-aware cell width for monospace/terminal rendering.
+unicode-width = "0.2"
 
 [dependencies.swash]
 version = "0.2.6"

src/shape.rs

@@ -237,6 +237,8 @@ fn shape_fallback(
             metadata: attrs.metadata,
             cache_key_flags: override_fake_italic(attrs.cache_key_flags, font, &attrs),
             metrics_opt: attrs.metrics_opt.map(Into::into),
+            // Set by the post-loop pass below once `end` is finalized.
+            terminal_cells: 1,
         });
     }
 
@@ -265,6 +267,19 @@ fn shape_fallback(
         }
     }
 
+    // Yoda: compute EAW terminal width per glyph now that `end` is finalized.
+    // We use the cluster's text (byte range into `line`) rather than just the
+    // first char so ZWJ emoji sequences collapse to a single cluster width.
+    // `unicode-width` returns 0 for combining marks / variation selectors, 2
+    // for CJK & most emoji, 1 for everything else including Arabic.
+    {
+        use unicode_width::UnicodeWidthStr;
+        for glyph in &mut glyphs[glyph_start..] {
+            let cluster = line.get(glyph.start..glyph.end).unwrap_or("");
+            glyph.terminal_cells = UnicodeWidthStr::width(cluster).min(2) as u8;
+        }
+    }
+
     // Restore the buffer to save an allocation.
     scratch.harfrust_buffer = Some(glyph_buffer.clear());
 
@@ -590,6 +605,12 @@ fn shape_skip_glyphs(
                         .letter_spacing_opt
                         .map_or(0.0, |spacing| spacing.0);
                 let attrs = attrs_list.get_span(start_run + chr_idx);
+                // Yoda: EAW width for terminal-aware monospace snap (see
+                // ShapeGlyph.terminal_cells doc).
+                let terminal_cells = {
+                    use unicode_width::UnicodeWidthChar;
+                    UnicodeWidthChar::width(codepoint).unwrap_or(1).min(2) as u8
+                };
 
                 ShapeGlyph {
                     start: chr_idx + start_run,
@@ -608,6 +629,7 @@ fn shape_skip_glyphs(
                     metadata: attrs.metadata,
                     cache_key_flags: override_fake_italic(attrs.cache_key_flags, font, &attrs),
                     metrics_opt: attrs.metrics_opt.map(Into::into),
+                    terminal_cells,
                 }
             }),
     );
@@ -644,6 +666,14 @@ pub struct ShapeGlyph {
     pub metadata: usize,
     pub cache_key_flags: CacheKeyFlags,
     pub metrics_opt: Option<Metrics>,
+    /// Yoda: Unicode East Asian Width of the source text covered by this glyph,
+    /// in terminal cells (0, 1 or 2 per `unicode_width::UnicodeWidthStr::width`).
+    /// Populated at shape time from `line[start..end]`. Consumed by
+    /// `layout_to_buffer` when `match_mono_width` is Some — wide chars (emoji,
+    /// CJK, fullwidth forms) snap to 2 cells instead of rounding based on the
+    /// fallback font's natural glyph advance, which was the upstream bug
+    /// breaking terminal column alignment.
+    pub terminal_cells: u8,
 }
 
 impl ShapeGlyph {
@@ -2903,9 +2933,21 @@ impl ShapeLine {
                                     0.0
                                 },
                             );
-                            if let Some(match_em_width) = match_mono_em_width {
+                            if let Some(mono_width) = match_mono_width {
-                                // Round to nearest monospace width
+                                // Yoda: use Unicode East Asian Width, stored at
-                                x_advance = ((x_advance / match_em_width).round()) * match_em_width;
+                                // shape time, as the authoritative cell count.
+                                // `terminal_cells` is 0 for combining marks /
+                                // variation selectors (they stay 0-advance),
+                                // 2 for CJK & emoji clusters, 1 for everything
+                                // else (including Arabic, Latin, etc.).
+                                //
+                                // This replaces the previous round-to-nearest
+                                // logic which produced variable cell widths
+                                // for fallback glyphs because it depended on
+                                // the glyph's natural advance in the fallback
+                                // font rather than on the source character's
+                                // terminal width spec.
+                                x_advance = f32::from(glyph.terminal_cells) * mono_width;
                             }
                             if hinting == Hinting::Enabled {
                                 x_advance = x_advance.round();