fix(sessions): keep named-pipe socket paths inside the macOS sun_path limit (NEXT-01)

The pipe name mxaccess-gateway-{pid}-session-{32hex} plus .NET's
CoreFxPipe_ prefix overflowed the 104-byte Unix-domain-socket path limit
under the default per-user macOS TMPDIR (~49 chars), so every test that
opened a real pipe threw ArgumentOutOfRangeException at pipe creation
unless TMPDIR=/tmp was exported. Rename to mxgw-{pid}-{sessionUid} (the
session guid hex without the session- prefix; worst-case 43 chars) and
shorten the three test-fixture names the same way. Uniqueness is
unchanged: gateway pid + full session guid. The worker receives the pipe
name via its launch command line, so mixed Server/Worker deploy SHAs are
unaffected. Docs updated in the same change (gateway.md,
GatewayProcessDesign, GatewayConfiguration, Sessions, CLAUDE.md); new
regression test pins the format and the length budget.

Verified: SessionManagerTests 39/39; SessionWorkerClientFactory,
GatewayEndToEndFakeWorkerSmoke, WorkerClient, and ReconnectReplay suites
33/33 under the default macOS TMPDIR — this also retires the
previously-misdiagnosed 'macOS pipe-timeout test failures': they were
this path-length throw, not a timeout-message defect.

docs/GatewayConfiguration.md

@@ -663,7 +663,7 @@ See each client README for the as-built behavior.
 
 Transport security here applies only to the public gRPC channel. The
 gateway↔worker link is a per-session **named pipe**
-(`mxaccess-gateway-{gatewayPid}-{sessionId}`), not a network socket. It is not
+(`mxgw-{gatewayPid}-{sessionUid}`), not a network socket. It is not
 TLS-encrypted and does not need to be: it never leaves the local Windows host and
 is secured by the OS pipe ACL. See [Worker Frame Protocol](./WorkerFrameProtocol.md).