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.

ports/mk/port.mk

@@ -0,0 +1,48 @@
+# SPDX-License-Identifier: Apache-2.0
+# Shared definitions for all Schunix ports.
+# Include this at the top of each port's Makefile:  include ../mk/port.mk
+#
+# Layout contract — a port directory contains ONLY hand-written files:
+#
+#   ports/<name>/Makefile      required
+#   ports/<name>/patches/*     optional
+#
+# Everything generated goes under $(PORTDIR) = build/ports/<name>/ :
+#
+#   build/ports/<name>/src     extracted upstream tarball
+#   build/ports/<name>/obj     out-of-tree build directory
+#   build/ports/<name>/prefix  install prefix (musl's headers/libs land here)
+#
+# If a file appears inside ports/<name>/ that you did not write by hand,
+# it is in the wrong place. `make clean` deletes $(PORTDIR) and nothing else.
+
+PORTSDIR  := $(abspath $(dir $(lastword $(MAKEFILE_LIST)))/..)
+TOPDIR    := $(abspath $(PORTSDIR)/..)
+
+SYSROOT   := $(TOPDIR)/sysroot
+DISTDIR   := $(TOPDIR)/dist
+BUILDROOT := $(TOPDIR)/build/ports
+
+# Port name is the directory name — no per-port boilerplate needed.
+PORT     := $(notdir $(CURDIR))
+PORTDIR  := $(BUILDROOT)/$(PORT)
+SRCDIR   := $(PORTDIR)/src
+BUILDDIR := $(PORTDIR)/obj
+PREFIX   := $(PORTDIR)/prefix
+
+# musl is the libc every other port links against; its install prefix is a
+# fixed location so ports don't need to know where musl's own source lives.
+MUSL     := $(BUILDROOT)/musl/prefix
+
+CC      := clang
+# FS register belongs to the kernel; never let the compiler touch %fs directly.
+CFLAGS_BT := -fno-stack-protector
+
+.PHONY: all download extract build install clean distclean
+all: install
+
+$(DISTDIR):
+	mkdir -p $(DISTDIR)
+
+$(SYSROOT)/bin:
+	mkdir -p $@