decouple agent version line from panel version

Agent display version now comes from the agent line's own source
(config.agentDisplayVersion injection + defaultAgentDisplayVersion
constant, currently A2.3.0) instead of being derived from the panel's
displayVersion. Masters resolve the agent line's latest from each
release's agent-version.txt marker (selfupdate.ResolveLatestAgentTarget,
legacy tag derivation as fallback for pre-marker releases): update
intents target the marker under the panel tag, panel-only releases
produce no intent, and the nodes page badges agent rows against
their own line via PanelUpdateInfo.displayLatestAgentVersion.

Windows agent frozen at A2.2.1: release discipline in docs/release.md
+ scripts/release-build.sh (agent assets reused verbatim when the
agent code is unchanged, marker always emitted, no Windows exe),
install-agent.ps1 pinned at V2.4.0, and the A2.2.1 protocol shapes
locked by TestAgentLegacyWindows221Contract.

Agent update tests now derive their targets from the live agent
line instead of a hardcoded A2.0.x the line has passed.

install-agent.ps1

@@ -27,8 +27,10 @@ param(
 
 $ErrorActionPreference = "Stop"
 
-# Release download base, same shape as install-agent.sh's
-# RELEASE_BASE (the version segment is bumped by the release flow).
+# Release download base. FROZEN at V2.4.0 — do not bump: the Windows
+# agent line ended at A2.2.1 and V2.4.0 is the last release that
+# contains the Windows exe (later releases ship no exe; see
+# docs/release.md, "Windows freeze").
 $ReleaseBase = "https://git.881218.xyz/rogue/3x-ui-agent/releases/download/V2.4.0"
 if (-not $AgentUrl) {
     $AgentUrl = "$ReleaseBase/x-ui-agent-windows-amd64.exe"