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.

docs/BUGS.md

@@ -146,6 +146,54 @@ and [B44](#b44--r8-broke-the-app-under-instrumentation-in-five-places).
 
 ## Fixed
 
+### B63 — a taskbar pin on Windows lost its icon
+
+**Symptom.** Proton Stream pinned to the Windows taskbar showed its icon at
+first, then after a while — reliably after an update — the generic program
+icon.
+
+**Cause.** Two, each enough on its own. `proton-stream.exe` carried no icon
+resource: the window sets one at runtime, which is what a pin shows while the
+icon cache still holds it, but a pin made from the running window points at
+the exe, and once the cache is rebuilt the exe has nothing to give. A pin made
+from the Start Menu shortcut instead took the shortcut's `Icon`, which Windows
+Installer keeps in `C:\Windows\Installer\{ProductCode}`. Every upgrade has a
+new ProductCode and deletes the old product's folder, icon included.
+
+**Fix.** `crates/pstr-app/build.rs` compiles `proton-stream.rc`, which embeds
+`packaging/windows/proton-stream.ico`, into the GUI exe: `rc.exe` on MSVC,
+mingw's `windres` on the cross build. Both installer definitions drop the
+shortcut's `Icon` (and the WiX one `Advertise`), so the shortcut shows the
+exe's icon too. A pin that already lost its icon keeps the dead reference;
+unpin and pin again once.
+
+**Verified.** `objdump -p` on the cross-built exe lists the `RT_ICON` entries,
+all seven sizes. `msiinfo export … Shortcut` on a wixl build has an empty
+`Icon_` column. Not yet checked on a Windows taskbar across an upgrade.
+
+### B62 — an MSI upgrade left the app without libmpv
+
+**Symptom.** Installing a new `.msi` over an old one ended with an app that
+would not start for want of mpv. Installing the same `.msi` a second time
+fixed it.
+
+**Cause.** Both installers remove the old product early — the wixl one at
+`RemoveExistingProducts` after `InstallInitialize`, WiX's `MajorUpgrade` by
+default after `InstallValidate` — but costing runs before either, at
+`CostFinalize`. Costing found the old `libmpv-2.dll` on disk with the same
+version as the new one, since mpv is pinned, and decided not to copy it. Then
+removing the old product deleted it. The second install found no DLL on disk,
+so it copied one.
+
+**Fix.** `REINSTALLMODE=amus` in both installer definitions, which copies every
+file whatever is on disk. Every file is private to the install directory, so
+the forced overwrite can downgrade nothing shared.
+
+**Verified.** `msiinfo export … InstallExecuteSequence` on a wixl build shows
+`FileCost` 900 and `CostFinalize` 1000 ahead of `RemoveExistingProducts` 1501,
+and `msiinfo export … Property` has `REINSTALLMODE amus`. Not yet checked as an
+upgrade on Windows.
+
 ### B61 — later seasons were captioned with season one's episode names
 
 **Symptom.** On a 42-title AniList library, Attack on Titan seasons 2–4 and