Initial commit: Schunix as it stands before the SchOS rename

An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.

No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.

tools/crashshot.ps1

@@ -0,0 +1,115 @@
+# Boot Schunix, crash it on purpose, and capture the crash screen as an image.
+#
+# The crash screen is the one part of the system you cannot look at by using
+# the system, so it is the one part that quietly rots. This drives it through
+# /proc/sysrq and takes a picture, which turns "does the panic screen still
+# render" into something answerable in thirty seconds.
+#
+# QEMU's monitor is on its own socket so the serial console stays free for the
+# shell. screendump writes a PPM; the PNG conversion is optional and skipped if
+# ImageMagick is not around.
+#
+#   tools\crashshot.ps1              crash the ramdisk-root build
+#   tools\crashshot.ps1 -SchFS       with a SchFS volume attached
+
+param(
+    [switch]$SchFS,
+    [int]$BootSeconds = 20,
+    [string]$Out = "build\crashscreen.ppm",
+    [string]$Qemu = "$env:USERPROFILE\scoop\apps\qemu\current\qemu-system-x86_64.exe"
+)
+
+$ErrorActionPreference = 'Stop'
+$root = Split-Path $PSScriptRoot -Parent
+$hda  = Join-Path $root 'schunix.img'
+$hdb  = if ($SchFS) { Join-Path $root 'build\schfs.img' } else { Join-Path $root 'build\fs.img' }
+$log  = Join-Path $root 'build\crashshot.log'
+$ppm  = Join-Path $root $Out
+$mon  = Join-Path $root 'build\qemu-mon.sock'
+
+Remove-Item -Force $log, $ppm -ErrorAction SilentlyContinue
+
+# The monitor goes to a TCP port rather than a unix socket: PowerShell can open
+# a TCP client without any extra machinery, and QEMU on Windows has no unix
+# sockets anyway.
+$monPort = 55555
+
+$psi = New-Object System.Diagnostics.ProcessStartInfo
+$psi.FileName = $Qemu
+$psi.Arguments = @(
+    "-drive file=`"$hda`",format=raw,index=0,media=disk"
+    "-drive file=`"$hdb`",format=raw,index=1,media=disk"
+    '-m 1G -smp 1 -cpu qemu64,+rdtscp -machine pc-i440fx-9.2'
+    '-display none'
+    "-chardev stdio,id=c0,logfile=`"$log`""
+    '-serial chardev:c0'
+    "-monitor tcp:127.0.0.1:$monPort,server,nowait"
+) -join ' '
+$psi.RedirectStandardInput = $true
+$psi.UseShellExecute       = $false
+
+Write-Host "==> booting" -ForegroundColor Cyan
+$p = [System.Diagnostics.Process]::Start($psi)
+
+function Send-Line($text) {
+    foreach ($ch in ($text + "`n").ToCharArray()) {
+        $p.StandardInput.Write($ch)
+        $p.StandardInput.Flush()
+        Start-Sleep -Milliseconds 30    # serial has no flow control
+    }
+}
+
+try {
+    Start-Sleep -Seconds $BootSeconds
+
+    # Put something on the display first. The whole design of the crash screen
+    # is that it dims what was there rather than clearing it, so a blank screen
+    # underneath would prove nothing.
+    Write-Host "  > filling the screen with normal output" -ForegroundColor DarkGray
+    Send-Line 'ls /bin'
+    Start-Sleep -Seconds 3
+    Send-Line 'cat /proc/cpuinfo'
+    Start-Sleep -Seconds 2
+
+    Write-Host "  > echo c > /proc/sysrq" -ForegroundColor DarkGray
+    Send-Line 'echo c > /proc/sysrq'
+    Start-Sleep -Seconds 4
+
+    Write-Host "==> capturing the framebuffer" -ForegroundColor Cyan
+    $client = New-Object System.Net.Sockets.TcpClient('127.0.0.1', $monPort)
+    $stream = $client.GetStream()
+    $writer = New-Object System.IO.StreamWriter($stream)
+    $writer.AutoFlush = $true
+    Start-Sleep -Milliseconds 500
+    # QEMU on Windows wants a native path here, not the one PowerShell shows.
+    $writer.WriteLine("screendump $ppm")
+    Start-Sleep -Seconds 2
+    $writer.WriteLine('quit')
+    Start-Sleep -Milliseconds 500
+    $client.Close()
+} finally {
+    if (-not $p.HasExited) { $p.Kill() }
+    Start-Sleep -Milliseconds 800
+}
+
+if (Test-Path $log) {
+    $text = Get-Content $log -Raw
+    Write-Host "==> serial transcript" -ForegroundColor Cyan
+    # Only the crash report, not the whole boot.
+    $idx = $text.IndexOf('Schunix stopped')
+    if ($idx -ge 0) { Write-Host $text.Substring($idx) } else { Write-Host $text }
+}
+
+if (Test-Path $ppm) {
+    $size = (Get-Item $ppm).Length
+    Write-Host "==> $ppm ($size bytes)" -ForegroundColor Green
+    $png = [System.IO.Path]::ChangeExtension($ppm, '.png')
+    $magick = Get-Command magick -ErrorAction SilentlyContinue
+    if ($magick) {
+        & magick $ppm $png
+        Write-Host "==> $png" -ForegroundColor Green
+    }
+} else {
+    Write-Host "FAIL: no screendump was produced" -ForegroundColor Red
+    exit 1
+}