feat: NAPALM driver for Windows over PowerShell Remoting

Talks PSRP over WinRM via pypsrp, one runspace pool per connection. Each
getter sends one PowerShell script that projects cmdlet results onto flat
fields and ends in ConvertTo-Json, so the Python side parses JSON, not text.

Covers facts, interfaces, IP addresses, ARP, routes and services, plus
service start/stop/restart/enable/disable. Service names are validated and
quoted as PowerShell verbatim strings, typographic quotes included.

Fixtures are synthetic: they pin down the JSON the scripts are designed to
emit. tools/harvest.py records the real output from a host.

Refs christianmanivong/netork#300

tests/fixtures/synthetic/README.md

@@ -0,0 +1,10 @@
+# Synthetic fixtures
+
+These files are what the driver's PowerShell scripts *emit*, not raw device
+output: each script projects its cmdlet results onto flat, primitive fields and
+ends in `ConvertTo-Json`. The shape is therefore this driver's own contract,
+and the Python side is tested against it here.
+
+What they cannot prove is that the scripts produce this shape on a real host
+(property names, Windows PowerShell 5.1 serialisation quirks). They are to be
+replaced by output recorded from a Windows Server and a Windows 11 client.