feat(lib): baseline JPEG decoder -- the browser shows photos

Adds a from-scratch baseline JPEG decoder (ITU-T T.81, SOF0 sequential DCT),
so the Web app now displays the web's dominant photo format alongside PNG and
BMP. image_decode dispatches on the JPEG magic, so the browser needs no change
beyond that.

lib/jpeg.c handles grayscale and YCbCr, arbitrary chroma subsampling
(4:4:4/4:2:2/4:2:0), restart markers, and multiple DQT/DHT tables. Huffman
uses canonical tables (counts + symbols, like the inflate decoder) to avoid a
large fast-table; the transform is the standard fixed-point integer row/column
IDCT; colour is integer YCbCr->RGB. All heap-allocated, no floating point, so
it is safe in the freestanding kernel. Progressive (SOF2) and arithmetic-coded
JPEGs are rejected rather than mis-decoded.

Verified on the host against PIL-encoded references before going in the kernel,
the same way the crypto and PNG decoders were: grayscale decodes at mean
absolute error 0.9/channel, 4:4:4 at 1.7 (pure IDCT rounding vs libjpeg), and
4:2:0 at 4.3 (nearest-neighbour chroma upsampling softens colour edges -- a
known, visually-fine artifact). In QEMU the Web app fetched a 400x260 JPEG over
HTTP and rendered it correctly -- gradient, an antialiased red circle, a green
box, a yellow diagonal. Builds clean on x86_64, aarch64 and armv8i.

Limits: baseline only (no progressive), and chroma upsampling is
nearest-neighbour rather than the smoother interpolation libjpeg uses. The
browser still shows images by URL, not inline in a page.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Fh5z6V7nGPbHvqVG5p4CXX

include/lib/jpeg.h

@@ -0,0 +1,11 @@
+#ifndef _LIB_JPEG_H
+#define _LIB_JPEG_H
+
+#include <stdint.h>
+#include <lib/image.h>
+
+// Baseline JPEG (SOF0) decoder. Fills img with a heap-allocated 0xAARRGGBB
+// buffer on success (return 0); progressive/arithmetic JPEGs are rejected.
+int jpeg_decode(const uint8_t* data, unsigned len, image_t* img);
+
+#endif // _LIB_JPEG_H