docs(distribution): Steam is the only channel — the zip, winget and the Store are dead

The site said "Coming soon to Steam" while the app docs still said direct download plus
winget primary with Steam parked. One of them had to move, and the creator's ruling is
Steam exclusive: a free app on Steam with the paid edition as a Steam DLC, and the
$10/month plan sold off-Steam by Polar.

Records the constraint that shapes the code, not just the prose:
- Steamworks requires in-app purchases to use the Steam Wallet (Microtransactions doc,
  quoted in commercial.md), so the monthly is *stated*, never *sold*, inside a Steam
  build; the lifetime is plain DLC bought on the store page and read with BIsDlcInstalled.
- Entitlement is `steamDlcOwned || polarKeyValid`, both resolving to one OutputProfile.
  steam_api64.dll ships beside the exe and is a functional dependency, not the rejected
  "IP protection" native DLL — without it DLC ownership cannot be read. Launched without
  Steam, the app falls back to the free profile rather than refusing to start.
- Signing is moot: Steam installs never meet SmartScreen. Artifact Signing stays on the
  shelf for the day a non-Steam download exists. Year-one fixed cost is ~$115.
- The exe rename must land before the first depot upload — Steamworks warns about
  renaming a live exe.

Distribution.md is rewritten to SteamPipe mechanics (publish output, depot, steamcmd,
DLC wiring, clean-account purchase/refund test). product.md, decisions.md (a new
"Steam exclusive" entry with every dead vector), TASKS.md (Steamworks entitlement row,
Steam packaging row), ai.md, index.md and the TASK 48 banner follow. MyMistakes.md gains
the Steam rules that cost real time today: installs skip SmartScreen, updates are free,
the Wallet rule, and the recurring-billing caveat.

MyMistakes.md

@@ -166,6 +166,29 @@ in Studio → Settings → Channel → **Feature eligibility**:
   *"Advanced YouTube features aren't available for this account"*. If ID verification is ever
   wanted on a personal channel, do it **before** converting to a brand account.
 
+### STEAM IS THE DISTRIBUTOR — AND WHAT IT DOES NOT ALLOW (2026-10-31)
+
+**Why recorded:** the distribution answer changed three times in one day (winget → zip+GitHub →
+Steam), and each pivot invalidated a doc written hours earlier. These are the facts that won't
+change.
+
+- **Steam installs skip SmartScreen entirely** — Valve's client delivers the files, so there is no
+  browser download, no Mark-of-the-Web and no publisher warning. An unsigned exe is *fine* in this
+  channel, and a certificate is only needed if a non-Steam download ever exists.
+- **Steam updates the app** — no self-updater, no version banner, no update endpoint.
+- **⛔ In-app purchases must use the Steam Wallet.** Steamworks, *Microtransactions (In-Game
+  Purchases)*: *"For any in-game purchases, you'll need to use the microtransaction API so Steam
+  customers can only make purchases from the Steam Wallet."* So a subscription sold through Polar
+  inside a Steam build is a policy violation. **Lifetime = plain DLC bought on the store page**
+  (the app only calls `BIsDlcInstalled`); the monthly is *stated*, never *sold*, in-app.
+- **Steam can technically do recurring billing** (`ISteamMicroTxn` + `ProcessAgreement`) but its own
+  doc opens with *"Recurring Subscriptions is not a fully supported feature"* and requires a server
+  of yours to poll renewals — more infrastructure than Polar, for a 30% cut.
+- **Renaming a live exe is painful** (Steamworks → *Tip – Renaming a live exe*). Do the
+  `LlamaCasty.exe` rename **before** the first depot upload, not after.
+- **The Steam store page must be public two weeks before release**, and the $100 Steam Direct fee
+  must be paid before the page can be configured at all.
+
 ### CONVERTING A MEDIAPIPE .tflite (WITH CUSTOM OPS) TO ONNX — AND PROVING IT (RECIPE, TASK 51)
 
 **Problem:** MediaPipe segmentation models end in the custom op `Convolution2DTransposeBias`;