fix(app,packaging): keep libmpv and the taskbar icon across Windows upgrades

An MSI upgrade left the app without libmpv-2.dll until it was installed a
second time (B62). Costing runs before RemoveExistingProducts, saw the old
DLL at the same version and skipped the copy, and removing the old product
then deleted it. Both installer definitions now set REINSTALLMODE=amus;
every file is private to the install directory.

A taskbar pin lost its icon after a while (B63). The exe carried no icon
resource, and the Start Menu shortcut's icon lived in the per-ProductCode
installer cache that each upgrade deletes. build.rs now compiles the .ico
into proton-stream.exe, and the shortcut has no Icon of its own (and in the
WiX build is no longer advertised), so both show the exe's icon.

packaging/README.md

@@ -124,7 +124,7 @@ The `.icns` is rasterized from `io.narl.proton-stream.svg` when `rsvg-convert` o
 | `io.narl.proton-stream.desktop` / `.svg` | Freedesktop entry and icon |
 | `macos/Info.plist.in` | `.app` bundle plist; `@VERSION@`/`@BIN@`/`@APP_ID@` substituted at build time |
 | `windows/proton-stream.wxs` | WiX v4+ installer definition |
-| `windows/proton-stream.ico` | MSI and shortcut icon, rasterized from the SVG |
+| `windows/proton-stream.ico` | Exe and Add/Remove Programs icon, rasterized from the SVG; `crates/pstr-app/build.rs` compiles it into `proton-stream.exe` |
 | `windows/license.rtf` | Fallback for the installer's license dialog; the repo's own `LICENSE` is preferred when it exists |
 
 The Debian `control` file has no template — `scripts/build.sh` writes it, because