feat(entitlement): IEntitlement seam — Steam DLC answers, everything fails closed

The first half of surge day 1. One question, "may this session emit Pro output?", with
one method on the interface so nothing downstream knows which rail proved it.

SteamEntitlement reads the lifetime SKU the way Valve intends: SteamAPI.Init() then
SteamApps.BIsDlcInstalled(dlcAppId). Four different failure paths all resolve to the
free tier rather than to Pro or to a crash — no DLC id configured, no Steam client,
steam_api64.dll missing (a throw on first touch), and simply not owned. A copied folder
run without Steam is the common case, not an exceptional one.

NotConfigured() is the shipping default until Steamworks creates the DLC app; it answers
false without ever touching the native library, so today's builds behave exactly as they
did before this commit.

HasProOutput is now IsPremium || DeveloperMode || rail, with an internal EntitlementOverride
test seam (internal, because a public setter here is a licence bypass). Four tests: no id
reads false, no client reads false without throwing, the fake reports what a test sets, and
the view model takes its answer from the rail.

Steamworks.NET 2024.8.0 added; the native DLL still comes from scripts/steam-sdk.sh, so the
csproj change is three lines and no formatting churn.

Distribution.md

@@ -97,9 +97,13 @@ version banner** — Steam is the update path, and that policy has not changed.
 
 ## 4. Entitlement wiring
 
-- `IEntitlement` has two implementations: **Steam DLC** (lifetime, local) and **Polar** (monthly,
-  network + 7–14 day lease). Resolution is `Pro = steamDlc || polarValid`; anything unresolved is
-  the free profile.
+- `IEntitlement` (`Services/Entitlement/`) has one method, `IsPro`. **Steam DLC** answers it today
+  (`SteamEntitlement`, `Steamworks.NET` + `steam_api64.dll`); the **Polar** monthly rail plugs in
+  beside it. Resolution is `Pro = steamDlc || polarLease`, then `MainViewModel.HasProOutput` adds
+  the Debug-only `DeveloperMode`; anything unresolved is the free profile.
+- **`SteamEntitlement.NotConfigured()` is the shipping default** until the DLC app id exists — it
+  answers false without touching the native library. Set the id when Steamworks creates the DLC.
+- **Fail closed on every path:** no id, no client, missing DLL, thrown load, not owned.
 - **Never sell the monthly inside the Steam build.** Steamworks requires in-app purchases to use
   the Steam Wallet; the app may *state* that monthly plans exist on the site, with no checkout,
   no price and no payment field. Confirm the wording with Steamworks support before shipping it.