Fix UDP port 9100 test race; correct CompositeDiscoveryService design doc

FIX 1 — UDP 9100 binding race: SprtDiscoveryServiceTests and
CompositeDiscoveryServiceTests both use exclusive UDP 9100 binds. xunit
runs test classes concurrently by default, causing the tests to fail
intermittently with AddressAlreadyInUse. Add [Collection("udp-9100")]
attribute to both classes so they run serially instead.

FIX 2 — Design doc: CompositeDiscoveryService does NOT forward the callback
down to children; it raises the caller's callback itself after all children
finish, once per unique IP. Rewrite the relevant sentence to describe the
real behavior.

Verify: 30 tests pass (x2), build clean, no production code touched.
Claude-Session: https://claude.ai/code/session_01E6N3P4nSZLtp4BC2LXVref

Inspectron.Epson.Tests/CompositeDiscoveryServiceTests.cs

@@ -2,6 +2,7 @@ using Inspectron.Epson;
 
 namespace Inspectron.Epson.Tests;
 
+[Collection("udp-9100")]
 public class CompositeDiscoveryServiceTests
 {
     private sealed class StubDiscovery : IDiscoveryService

Inspectron.Epson.Tests/SprtDiscoveryServiceTests.cs

@@ -3,6 +3,7 @@ using Inspectron.Epson;
 
 namespace Inspectron.Epson.Tests;
 
+[Collection("udp-9100")]
 public class SprtDiscoveryServiceTests
 {
     // The exact 39 bytes the printer returned on the bench:

docs/superpowers/specs/2026-09-03-pos895-integration-design.md

@@ -67,7 +67,9 @@ this service.
 
 **`CompositeDiscoveryService`** - takes `IEnumerable<IDiscoveryService>`, runs
 them concurrently on the shared timeout, merges results by `IPAddress` with
-first-wins, and forwards each child's `onPrinterDiscovered` callback as it fires.
+first-wins. It does not pass the `onPrinterDiscovered` callback to its children;
+instead, it raises the caller's callback itself after all children finish,
+once per unique IP, so a printer answering two discovery protocols notifies only once.
 It absorbs a single child's failure (logs it, keeps the other children's
 results) but rethrows when every child fails, since that means discovery
 itself is broken rather than that the network simply has no printers on it.