docs(migration): §3.1 — single-adapter adopters skip PlatformRouter

PlatformRouter is the right primitive for heterogeneous platforms
behind one process. Single-adapter adopters (salesagent today —
GAM-only deployments, with Kevel/Broadstreet/Triton/Xandr being
unused scaffolding) skip the router entirely: instantiate one
GAMPlatform, pass to serve(), let multi-tenancy ride on
Account.metadata['tenant_id'].

For that shape the migration is "delete the registry, instantiate
one GAMPlatform," not "translate registry into router." Adding a
"Who needs this" callout at the top of §3.1 so single-adapter
adopters don't waste time on the router pattern they don't need.

Also fixes a verbatim duplicate paragraph at the end of §3.1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bokelley

examples/multi_platform_seller/MIGRATION_FROM_ADAPTER_REGISTRY.md

@@ -202,6 +202,21 @@ an aspirational re-skin of the existing surface.
 
 ### 3.1 `ADAPTER_REGISTRY` → `PlatformRouter`
 
+**Who needs this.** `PlatformRouter` is the right primitive when
+you have **heterogeneous platforms behind one process** —
+different tenants routed to different `DecisioningPlatform`
+implementations (GAM for tenant A, Kevel for tenant B, etc.).
+**Single-adapter adopters skip the router**: instantiate one
+`GAMPlatform` (or `KevelPlatform`, or whichever), pass it directly
+to `serve(...)`, and let multi-tenancy ride on
+`Account.metadata['tenant_id']` per-request. Salesagent's actual
+deployments are GAM-only today (Kevel/Broadstreet/Triton/Xandr
+adapters exist but have no client traffic); for that shape the
+migration is "delete the registry, instantiate one `GAMPlatform`,"
+not "translate registry into router." Keep reading §3.1 if you
+genuinely run more than one upstream behind the same process; jump
+to §3.2 if you don't.
+
 **Before** — `salesagent/src/adapters/__init__.py:17`:
 
 ```python
@@ -258,11 +273,6 @@ request. Connection pools, OAuth token caches, and any platform-level
 state amortise across the platform's lifetime — the per-request
 instantiation overhead in the registry pattern goes away.
 
-Platforms are constructed once, at process start, and reused for every
-request. Connection pools, OAuth token caches, and any platform-level
-state amortise across the platform's lifetime — the per-request
-instantiation overhead in the registry pattern goes away.
-
 ### 3.2 `AdServerAdapter` ABC → `DecisioningPlatform` + `SalesPlatform`
 
 **Before** — `salesagent/src/adapters/base.py:174` and the Kevel