Give the file name the row, and stop repeating the project

On a real project the name lost the width fight and the trailing text read
"MyGame.Runtime MyGame.Runtime" — the directory and the project were the
same string, printed twice.

- The detail text now has a shrink factor of 1000 against the name's 1, plus
  a 45% cap, so it gives up its width first and the name is the last thing to
  ellipsize.
- A directory that already begins with the project name says everything the
  project name would, so only one of the two is shown.
- verticalTabs.showDirectory became never / duplicates / always, defaulting
  to duplicates: width goes to a directory only when two open tabs share a
  file name, which is the usual reason to want it. Turning showProject off
  as well gives the name the whole row.
- The trailing text is composed in the extension now rather than the webview,
  so the deduplication is unit-testable and there is one implementation.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

package.json

@@ -111,12 +111,22 @@
         "verticalTabs.showProject": {
           "type": "boolean",
           "default": true,
-          "description": "Show the containing project name beside the file."
+          "description": "Show the project name beside the file. The row's left border already encodes the project, so turning this off gives the file name the whole row."
         },
         "verticalTabs.showDirectory": {
-          "type": "boolean",
-          "default": true,
-          "description": "Show the containing directory beside the file, for files with the same name."
+          "type": "string",
+          "enum": [
+            "never",
+            "duplicates",
+            "always"
+          ],
+          "enumDescriptions": [
+            "Never show the directory. The file name gets the whole row.",
+            "Only when two open files share a name, which is the usual reason to need it.",
+            "Always show the directory."
+          ],
+          "default": "duplicates",
+          "description": "When to spend row width on the containing directory."
         },
         "verticalTabs.colorByProject": {
           "type": "boolean",