build(steam): stage steam_api64.dll beside the exe — one file, not the whole SDK

The SDK zip is 24 MB of headers, examples and tools we never build. The runtime
dependency is one native library, so scripts/steam-sdk.sh extracts exactly
sdk/redistributable_bin/win64/steam_api64.dll into steam/ (gitignored, like ffmpeg/ —
the script is the audit trail) and writes steam_appid.txt = 480, Valve's Spacewar app,
so SteamAPI.Init() resolves an app id locally before LlamaCasty has one of its own.

Two csproj details that would have cost a debugging session:
- Link="steam_api64.dll" puts it BESIDE the exe; without Link, None items keep their
  relative path and it lands in steam/, where the loader does not look.
- ExcludeFromSingleFile, for the same reason ffmpeg needed it: a native DLL otherwise
  gets swallowed into the single-file bundle and cannot be loaded at runtime.

steam_appid.txt is Debug-only — Valve's guidance is not to ship it — and the publish
output confirms it: dll present at the root, no appid file, exe still 127 MB (proof the
DLL was not bundled).

Also queued, as asked: preview undo (Ctrl+Z / Ctrl+Shift+Z) in edit mode, noted as the
partner to the defaults-vs-current split — undo covers the slip, defaults cover the mess.

Distribution.md

@@ -46,9 +46,12 @@ dotnet publish ytLive.csproj -c Release -r win-x64 --self-contained true \
    the DLC app ID is what the app passes to `BIsDlcInstalled`.
 3. **Depot.** One depot holding the publish output; the base app and the DLC share the install
    directory, and the DLC package grants no files of its own — it is entitlement only.
-4. **Steamworks SDK.** Drop `steam_api64.dll` (+ `steam_appid.txt` for local runs) into the publish
-   output; the managed side is a thin wrapper (`Steamworks.NET` or hand-written interop) behind
-   `IEntitlement`.
+4. **Steamworks SDK.** `./scripts/steam-sdk.sh ~/Downloads/steamworks_sdk_165.zip` extracts
+   `steam_api64.dll` from `sdk/redistributable_bin/win64/` into `steam/` (gitignored) and stages
+   `steam_appid.txt` = 480 — Valve's Spacewar app, so `SteamAPI.Init()` works locally before
+   LlamaCasty has an App ID. The csproj copies the DLL **beside the exe** (`Link`) with
+   `ExcludeFromSingleFile`, and copies `steam_appid.txt` in Debug only — never into a depot. The
+   managed side is the `Steamworks.NET` package (`SteamApps.BIsDlcInstalled`).
 5. **Store page** configured and made public **two weeks before release** — Valve's rule, and it is
    the window where wishlists accumulate.