Remove the FoundationNetworking re-export; correct the C3 row

Two fixes to the dissolved-shell surface.

ARCH-LAYER-007: the umbrella carried a conditional @_exported import of
FoundationNetworking, which no main target may import at any layer. Confirmed by
the linter rather than by reading - it is exactly the shape a per-file scan misses,
since the import lives in an umbrella exports file behind an @_exported and a
canImport guard. Removing it is safe because nothing consumes this package: a
proven zero across 479 manifests and 2329 dependency edges, and a source-level
import cannot compile without a manifest dependency, so that zero is sound rather
than an upper bound. Linter findings 17 -> 16, the finding gone, summary line
confirming the instrument ran over 6 files under 93 rules.

The README's dissolution table still recorded C3 (.mainEventLoopGroup) as stays

Author: coenttb

README.md

@@ -11,7 +11,7 @@
 > |---|---|
 > | `\.date` + `Date.Generator` (C1) | [swift-clocks-dependencies](https://github.com/swift-foundations/swift-clocks-dependencies) |
 > | `\.httpClient` (C2) | **deleted** — dead code (de-engine R3) |
-> | `\.mainEventLoopGroup` (C3) | stays here until β-end (de-engine R5) |
+> | `\.mainEventLoopGroup` (C3) | [swift-event-loop-group-dependencies](https://github.com/swift-foundations/swift-event-loop-group-dependencies) |
 > | console logger handler (C4) | [swift-logger-handlers](https://github.com/swift-foundations/swift-logger-handlers) `Logger.Handler.Console` |
 > | `InMemoryStore` TTL cache (C5) | [swift-time-to-live](https://github.com/swift-foundations/swift-time-to-live) `Store.InMemory` |
 > | `URL.canonical` (C6) | [swift-uri](https://github.com/swift-foundations/swift-uri) `RFC_3986.URI.canonical(host:)` |

Sources/ServerFoundation/exports.swift

@@ -20,7 +20,3 @@
 @_exported import Throttling_Dependencies
 @_exported import Translating_Dependencies
 @_exported import URLRequestHandler
-
-#if canImport(FoundationNetworking)
-    @_exported import FoundationNetworking
-#endif