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.

include/qr.h

@@ -0,0 +1,42 @@
+#pragma once
+
+// A QR encoder, for getting a crash report off a machine whose network stack is
+// dead.
+//
+// A panic report is worth nothing if it cannot leave the machine, and at panic
+// time there is no network, no filesystem write that can be trusted, and often
+// no second machine on the desk. A phone camera is a working out-of-band
+// channel that needs no drivers on our side at all. See docs/roadmap.md Phase 8.
+//
+// Deliberately small, in three ways:
+//
+//   - Byte mode only. The report is arbitrary bytes; alphanumeric mode would
+//     pack tighter but only for a restricted character set.
+//   - Error correction level L only. The screen is a clean, evenly lit,
+//     high-contrast source -- the failure mode QR's stronger levels protect
+//     against (dirt, print damage, curved packaging) does not apply.
+//   - Versions 1 to 6 only. Version 7 is where QR starts requiring an extra
+//     18-bit version-information block in two more places on the symbol, and
+//     version 10 is where the character count field grows and the alignment
+//     pattern grid becomes 2-dimensional. Stopping at 6 avoids both, and
+//     134 bytes is a large crash report.
+//
+// If a payload ever needs more than 134 bytes, the honest fix is to shorten the
+// report -- not to reach for version 10 and its two extra tables.
+
+#define QR_MAX_VERSION 6
+#define QR_MAX_SIZE    (17 + 4 * QR_MAX_VERSION)   // 41 modules per side
+#define QR_MAX_BYTES   134
+
+struct qr {
+  int size;                              // modules per side; 0 if unencoded
+  unsigned char mod[QR_MAX_SIZE][QR_MAX_SIZE];   // 1 = dark
+  unsigned char fn[QR_MAX_SIZE][QR_MAX_SIZE];    // 1 = function pattern
+};
+
+// Encode len bytes into q. Picks the smallest version that fits.
+// Returns 0, or -1 if len > QR_MAX_BYTES.
+int qr_encode(struct qr *q, const unsigned char *data, int len);
+
+// Largest payload, in bytes, for a given version (1..QR_MAX_VERSION).
+int qr_capacity(int version);