Add configurable ambiguous-width policy (narrow/wide) (#19864)

This PR introduces `compatibility.ambiguousWidth` as a **global**
compatibility setting (`narrow` default, `wide` optional).

The default remains `narrow`.

Why global-only in this PR:
- Width detection is currently process-wide (`CodepointWidthDetector`
  singleton).
- True profile-level ambiguous-width behavior would require broader
  architectural changes and is intentionally deferred to a follow-up
  design/PR.

What this PR guarantees:
- Terminal-side handling is consistent end-to-end for the selected
  ambiguous-width policy (rendering path + ConPTY/host propagation).

Known limitation:
- Some client applications (for example PSReadLine/readline-based apps)
  may still compute character widths independently.
- In such cases, cursor movement or Backspace behavior can differ from
  visual cell width even when terminal-side policy is consistent.

This is a compatibility/readability trade-off feature:
- `narrow`: prioritize cross-application compatibility.
- `wide`: prioritize readability with many CJK fonts.

Closes #153
Closes #370
Refs #2928
Refs #2049, #2066, #2375, #900, #5910, #5914

Co-authored-by: Leonard Hecker <lhecker@microsoft.com>

doc/cascadia/profiles.schema.json

@@ -2407,6 +2407,14 @@
             "console"
           ]
         },
+        "compatibility.ambiguousWidth": {
+          "default": "narrow",
+          "description": "Controls the cell width of East Asian Ambiguous characters.",
+          "enum": [
+            "narrow",
+            "wide"
+          ]
+        },
         "copyFormatting": {
           "default": true,
           "description": "When set to `true`, the color and font formatting of selected text is also copied to your clipboard. When set to `false`, only plain text is copied to your clipboard. An array of specific formats can also be used. Supported array values include `html` and `rtf`. Plain text is always copied.",