NormalizeNode re-selected ports whenever the request merely CONTAINED
port/serverId keys; the browser form always submits them (port=null in
random mode), so every save re-rolled both entry and exit ports. Now an
assigned port only moves on an explicit addressing change: mode switch to
random, a different fixed port value, or a server move where the port is
taken (fixed mode surfaces the 409). random->fixed pins the current port.
Regression: portlock_test.go (UI-shape payload saved twice, rename-only,
mode switches, fixed change/conflict, server move free/taken).
Golden re-captured: diff confined to nodePatchUiStyle/nodePatchJobs/
nodesAfterPatch/backup port leaves returning to the seed ports.
