feat: add --offline and bound the connect time

--offline (or RETROBIOS_INSTALL_OFFLINE=1, which the runs the
full-screen interface starts inherit) makes no network request. The
file list stops the run at once with an error event of reason
offline unless --manifest or --from-pack gives it, a file the run
would download fails at once with its own advice, and board detection
is skipped. --target and --list-targets refuse it. RETROBIOS_OFFLINE
is not reused: CI and the test suite set it for every process, and the
installer tests serve their files over loopback.

A connection now has 4 s to open, then the read keeps its 30 or 60 s
timeout: three tries that never connect end in about 15 s instead of
93, and a slow but open transfer is not cut.

A --dry-run prints its totals first. A plan taller than the screen
names the ten largest systems, and --verbose lists every system and
file. Events are unchanged.

install.ps1

@@ -11,7 +11,7 @@
 
     $ErrorActionPreference = "Stop"
     $defaultInstallUrl = "https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-    $defaultInstallSha256 = "240faa427aaa602deca1a3180f770c6f335095b1cf1d5bb28fc060eb55f54917"
+    $defaultInstallSha256 = "ea2777500114b796bb4a23ea9e0dd0a15bc4499bb1289e6e426943beece48865"
     $maximumInstallerBytes = 2MB
     $installerExit = 0
     $installer = if ($PSScriptRoot) { Join-Path $PSScriptRoot "install.py" } else { $null }