v0.3: resumable downloads with HTTP Range, Polly retry, persistent state

This is the robustness layer needed for real 14 GB builds. A 99%-complete
download that gets interrupted no longer means re-downloading 14 GB.

DownloadManager
---------------
- Range: bytes={resumeFrom}- on every (re)attempt; reads resumeFrom from
  the actual size of the .partial file so retries always resume from real
  on-disk state, not from a remembered counter.
- If-Range: ETag (or Last-Modified fallback). When the server returns 200
  instead of 206 we know the resource changed under us, so we discard
  .partial and start fresh.
- Polly resilience pipeline: 6 retries, exponential 1-32s with jitter,
  on HttpRequestException / IOException / TimeoutException / 5xx / 408 /
  429. Each retry re-evaluates resumeFrom from disk, so the server is
  asked only for what's actually missing.
- state.json persisted every 5s OR every 100 MiB, whichever comes first,
  via atomic write-then-rename. Holds url, total, downloaded, sha256,
  etag, last-modified, and the .partial path.
- Disk-space check happens once at fresh-start (1.05x expected size); a
  resume doesn't redo it.
- On final success: SHA-256 of the assembled .partial verified, then
  atomic rename to .zip and state.json deleted.

DownloadStateStore
------------------
- New IDownloadStateStore in PSLauncher.Core/Downloads.
- Stores under %LocalAppData%/PSLauncher/downloads/.
- Save / Load / Discard / ScanResumable. Tolerates malformed state files
  by ignoring them.

UI hint
-------
VersionRowViewModel now has ResumableBytes; when > 0, the install button
label switches to "↻  Reprendre (X%)" computed from
ResumableBytes / Remote.Download.SizeBytes. MainViewModel.RebuildList
queries IDownloadManager.GetResumableState(version) for each remote-only
row and populates ResumableBytes. Both the featured hero card and the
compact rows bind to InstallButtonLabel.

API change
----------
IDownloadManager gains GetResumableState(version) and
DiscardResumableState(version) so callers (and the UI) can reason about
in-progress downloads without poking at the filesystem directly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/PSLauncher.App/ViewModels/VersionRowViewModel.cs

@@ -37,6 +37,24 @@ public sealed partial class VersionRowViewModel : ObservableObject
     [ObservableProperty] private double _progressPercent;
     [ObservableProperty] private string? _progressDetail;
 
+    [ObservableProperty]
+    [NotifyPropertyChangedFor(nameof(InstallButtonLabel))]
+    [NotifyPropertyChangedFor(nameof(HasResumableDownload))]
+    private long _resumableBytes;
+
+    public bool HasResumableDownload => ResumableBytes > 0;
+
+    public string InstallButtonLabel
+    {
+        get
+        {
+            if (!HasResumableDownload) return "⬇  Installer";
+            if (Remote is null || Remote.Download.SizeBytes <= 0) return "↻  Reprendre";
+            var pct = (double)ResumableBytes / Remote.Download.SizeBytes * 100.0;
+            return $"↻  Reprendre ({pct:0}%)";
+        }
+    }
+
     public bool IsInstalled => State is VersionRowState.InstalledIdle or VersionRowState.Uninstalling;
     public bool IsRemoteOnly => State is VersionRowState.AvailableIdle;
     public bool IsBusy => State is VersionRowState.Downloading or VersionRowState.Installing or VersionRowState.Uninstalling;