feat(entitlement): pivot to paid app ownership

Check Steam ownership of the paid app with BIsSubscribedApp; keep Debug fake ownership out of Release. Ref: https://partner.steamgames.com/doc/api/ISteamApps#BIsSubscribedApp

docs/ai/product.md

@@ -199,9 +199,9 @@ Live channel policy, the artifact and costs: [`commercial.md`](commercial.md). B
 mechanics: [`Distribution.md`](../../Distribution.md). Retired routes and cause of death:
 [`decisions.md`](decisions.md).
 
-**One vector: Steam.** A free app on Steam with the paid edition as a Steam **DLC**; the monthly
-plan is sold outside Steam and is never purchasable inside a Steam build (Steamworks requires the
-Steam Wallet for in-app purchases). No direct download, no winget, no Microsoft Store.
+**One vector: Steam.** One paid app plus its associated free Demo. They use the same code; the Demo
+checks ownership of the paid app to unlock higher output quality. No direct download, no GitHub
+binary, no winget, no Microsoft Store, and no subscription.
 
 **Consequences that are architecture, not packaging detail:**
 
@@ -209,7 +209,7 @@ Steam Wallet for in-app purchases). No direct download, no winget, no Microsoft
   app keeps nothing that downloads or executes, for the same Store-policy reason it always had.
   `ytLive.Tests/StorePackagingTests.cs` must keep asserting that no self-updater returns.
 - **⛔ `steam_api64.dll` is a functional dependency**, not the rejected "native helper DLL for IP
-  protection" — Steam DLC ownership cannot be read without it. The app must start without Steam
+  protection" — Steam app ownership cannot be read without it. The app must start without Steam
   running and fall back to the free profile.
 - **The MSIX package stays buildable but is now unused** — `scripts/package-msix.sh` (TASK 48
   slice 2) still produces `artifacts/LlamaCasty.msix` from `Packaging/Package.appxmanifest` +
@@ -265,11 +265,10 @@ a non-Steam download exists:
 
 ### Distribution and licensing are independent — the channel never decides billing
 
-The Steam build sells the lifetime as **Steam DLC** and reads it with `BIsDlcInstalled`, while the
-monthly plan is billed by **Polar** outside Steam. Steamworks forbids taking payment inside the app
-with anything but the Steam Wallet, so the app states where the monthly lives and never sells it.
-The lesson worth keeping: the licensing choice rides on a **monetization** decision, not on a
-distribution decision.
+Steam distributes the paid app and its associated free demo. They use the same code; the demo asks
+Steam whether the current account owns the paid app, and the answer controls access to higher output
+quality. The paid app is a one-time purchase: $79 during the announced launch period, then $99 on
+the announced date. The app's EULA must expressly allow commercial and monetized streaming use.
 
 ### ⛔ "Only when the user says so" is enforced by Windows, not by us
 
@@ -311,4 +310,3 @@ nothing. COPPA is very unlikely to bind (not child-directed), but note the enfor
 (FTC Sept 2025 *Apitor*: operators are responsible for what third-party components collect) —
 so the answer is a documented inquiry, and ours is "no third-party analytics or ad SDKs, no
 persisted chat or reward payloads."
-