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.

run.ps1

@@ -0,0 +1,113 @@
+# Schunix launcher (Windows side).
+#
+# Why this exists: without it you have to hand-type a QEMU command line, and a
+# hand-typed one drifts -- notably onto a stale fs.img, which boots to
+# "fsinit: invalid file system" because the wrong disk is attached as hdb.
+#
+# The build no longer goes through WSL. WSL2 runs a real Linux kernel inside a
+# Hyper-V VM, so it hard-requires virtualisation, and enabling that puts the
+# Hyper-V hypervisor underneath the whole machine -- which makes VMware run
+# nested on top of it and slows the box down permanently. Not a trade worth
+# making for a build environment.
+#
+# Instead it builds natively: clang/lld cross-compile the kernel to bare-metal
+# ELF, mingw-w64 builds the host tools, and Git Bash supplies make, perl, sed
+# and dd. See the Toolchain section of the Makefile.
+#
+#   .\run.ps1              build (native) then boot, windowed
+#   .\run.ps1 -NoBuild     boot whatever is already built
+#   .\run.ps1 -Headless    no window; serial captured to build\serial.log
+#   .\run.ps1 -Gdb         wait for a debugger on :26000 before executing
+
+#   .\run.ps1 -SchFS       boot the ramdisk-root kernel with a SchFS volume
+#                          attached as hdb; then run  cat /proc/schfs
+
+param(
+    [switch]$NoBuild,
+    [switch]$Headless,
+    [switch]$Gdb,
+    [switch]$SchFS,
+    [string]$Qemu   = "$env:USERPROFILE\scoop\apps\qemu\current\qemu-system-x86_64.exe"
+)
+
+$ErrorActionPreference = 'Stop'
+$root = $PSScriptRoot
+
+# The two images QEMU needs. These paths are the single source of truth --
+# keep them in sync with the 'qemu' and 'qemu-schfs' targets in the Makefile.
+$hda = Join-Path $root 'schunix.img'
+$hdb = if ($SchFS) { Join-Path $root 'build\schfs.img' }
+       else        { Join-Path $root 'build\fs.img' }
+
+if (-not $NoBuild) {
+    Write-Host '==> building (native, no WSL)' -ForegroundColor Cyan
+
+    # The three toolchain directories, prepended so the build sees them whether
+    # or not the current shell happened to inherit them: scoop's shims (make),
+    # mingw-w64 (host gcc), and LLVM (clang, ld.lld, llvm-objcopy/objdump).
+    $bash = Join-Path $env:ProgramFiles 'Git\bin\bash.exe'
+    if (-not (Test-Path $bash)) { throw "Git Bash not found at $bash" }
+    $toolPath = @(
+        "$env:USERPROFILE\scoop\shims"
+        "$env:USERPROFILE\scoop\apps\mingw\current\bin"
+        "$env:ProgramFiles\LLVM\bin"
+    ) -join ';'
+    $env:PATH = "$toolPath;$env:PATH"
+
+    # -SchFS needs the ramdisk-root kernel: the root filesystem then comes out
+    # of memory, which leaves device 1 free for the SchFS volume. Booting the
+    # normal kernel with a SchFS disk as hdb would leave it with no root.
+    $target = if ($SchFS) { 'make ramroot && make build/schfs.img' } else { 'make' }
+    & $bash -lc "cd /d/DEV/Schunix && $target"
+    if ($LASTEXITCODE -ne 0) { throw "build failed (exit $LASTEXITCODE)" }
+}
+
+foreach ($img in @($hda, $hdb)) {
+    if (-not (Test-Path $img)) { throw "missing image: $img  (run without -NoBuild)" }
+}
+
+# Guard against the exact failure this script exists to prevent: booting against
+# a stale or empty hdb. Each format has its own magic in its own place.
+$fs = [System.IO.File]::OpenRead($hdb)
+try {
+    $buf = New-Object byte[] 4
+    # v1: superblock in block 1 with BSIZE 1024, FSMAGIC 0x10203040.
+    # SchFS: first uberblock in block 1 with SCHFS_BLKSIZE 4096, magic "SchF".
+    $null = $fs.Seek($(if ($SchFS) { 4096 } else { 1024 }), 'Begin')
+    $null = $fs.Read($buf, 0, 4)
+} finally { $fs.Close() }
+
+if ($SchFS) {
+    # 0x53636846 little-endian on disk: 46 68 63 53  ("FhcS" byte order).
+    if ($buf[0] -ne 0x46 -or $buf[1] -ne 0x68 -or $buf[2] -ne 0x63 -or $buf[3] -ne 0x53) {
+        throw "$hdb is not a valid SchFS image (bad uberblock magic). Rebuild it: make build/schfs.img"
+    }
+} else {
+    if ($buf[0] -ne 0x40 -or $buf[1] -ne 0x30 -or $buf[2] -ne 0x20 -or $buf[3] -ne 0x10) {
+        throw "$hdb is not a valid SCHFS image (bad superblock magic). Rebuild it: rm -f build/fs.img && make"
+    }
+}
+
+$qemuArgs = @(
+    '-drive', "file=$hda,format=raw,index=0,media=disk"
+    '-drive', "file=$hdb,format=raw,index=1,media=disk"
+    '-m', '1G'
+    '-smp', '2'
+    '-cpu', 'qemu64,+rdtscp'
+    '-machine', 'pc-i440fx-9.2'
+)
+
+if ($Headless) {
+    $log = Join-Path $root 'build\serial.log'
+    $qemuArgs += @('-display', 'none', '-serial', "file:$log")
+    Write-Host "==> headless; serial -> $log" -ForegroundColor Cyan
+} else {
+    $qemuArgs += @('-vga', 'std', '-serial', 'stdio')
+}
+
+if ($Gdb) {
+    $qemuArgs += @('-gdb', 'tcp::26000', '-S')
+    Write-Host '==> paused for gdb on localhost:26000' -ForegroundColor Yellow
+}
+
+& $Qemu @qemuArgs