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.

.gitignore

@@ -21,3 +21,7 @@ CREDENTIALS.md
 .aider*
 ffmpeg/
 BuildCounter.txt
+
+# Steamworks runtime (steam_api64.dll, 312 KB) — extracted from the SDK zip, never committed;
+# scripts/steam-sdk.sh documents which file and why. Same rule as ffmpeg/: the pin is the audit trail.
+steam/