Repo and compare browsers stat every entry on the file system on every frame (plus O(n²) Contains lookups) #498

What's wrong

Since #482/#490 ("Classify browser entries from the repository root"), the browsers decide whether an entry is a folder by asking the file system (RepoBrowsing.IsDirectory → Directory.Exists) instead of looking at how the entry is spelled. That is the correct check, but it runs again on every frame rather than once when the listing is taken:

• Compare browser. ShowCompareBrowser (ProjectDirector/ProjectDirector.cs:2074-2080) does the following every frame:

  • builds BrowserContentsBase.Union(BrowserContentsCompare)
  • calls ListBrowserDirectories(entries, baseRoot, compareRoot)
  • then calls Except(directories)

  IsDirectory (ProjectDirector/RepoBrowsing.cs:52-53) calls Directory.Exists once per root. A plain file therefore costs a failed stat in both repositories on every frame.

• Repo browser. ShowRepoBrowser (ProjectDirector.cs:2242-2247) does the same for the base repository.

• Per-row lookups. Each compare-browser row also calls BrowserContentsBase.Contains(path) and BrowserContentsCompare.Contains(path) (around ProjectDirector.cs:2112-2113 and 2182-2183). These are linear scans of a Collection<T>, so each frame does O(n²) work.

The listing only changes in SwitchRepoBrowserPath and SwitchCompareBrowserPath (ProjectDirector.cs:2439-2471). Those are the only places the classification can change.

Why it matters

The browser is drawn whenever a repository is selected. Comparing a folder of 150 entries against a sibling repository costs about 300 stat calls per frame, which is about 18,000 per second at 60 fps.

Each call gets noticeably more expensive in directory-heavy trees, and on Windows with real-time antivirus scanning. The UI can then stutter just from showing a folder.

The listing also isn't refreshed after Give/Take/X (#456), so re-checking every frame doesn't even keep it current.

Suggested fix / acceptance criteria

• Classify entries once, when the listing is taken. RepoBrowsing.List or the Switch*BrowserPath methods would produce three things, stored next to BrowserContentsBase and BrowserContentsCompare:
  • the folders
  • the files
  • which side each entry is on, as one HashSet<RelativePath> per side
• ShowRepoBrowser and ShowCompareBrowser read only the cached sets. They make no file-system calls and no linear Contains lookups while drawing.
• After a Give/Take/X action, take the listing again, which re-classifies it. This also fixes the stale-row half of Compare browser: Give/Take and X only respond on the first row that shows them, and a row that was acted on stays listed until you navigate away #456.
• RepoBrowsingTests and BrowserDirectoryTests still pass.
• A new test checks that the classification made at listing time matches IsDirectory for:
  • nested folders
  • a name that is a folder in one repository and missing in the other