Replace vvfat SD with browser-built raw FAT32 from OPFS

Browser SD files are persisted in OPFS (web/sdstore.js) and edited in the
Files tab. At boot the browser builds a deterministic 64 MiB FAT32 image
(web/fat32.js) with nested directories and VFAT long names, then mounts
it as a raw block device, bypassing QEMU's broken populated-vvfat WASM
coroutine path. Guest writes mutate only the ephemeral image.

Exposes a hidden #frame-generation indicator that increments on every
e-ink flush, so automation can wait on real display updates instead of
fixed sleeps. Adds tests/ with a Makefile runner: 'make test' for JS +
FAT32/mtools checks, 'make browser-test' for a headless-Firefox boot of
the official CrossPoint 1.4.1 firmware that navigates to
Test/example.txt. Bumps third_party/qemu to the SPI-SD/CMD13 fixes and
updates README/ROADMAP/ISSUES for the new architecture.

scripts/test-web.mjs

@@ -1,5 +1,6 @@
 import assert from 'node:assert/strict';
 import { BUTTONS, DEFAULT_SD_FILES, EXAMPLE_SD_TEXT, formatBytes, setButton, mergeFirmware } from '../web/app.js';
+import { buildFat32Image, FAT32_IMAGE_SIZE } from '../web/fat32.js';
 
 // Merge - app image is placed at 0x10000 in a 0xFF-erased 16 MB flash with bootloader and partitions.
 const bootloader = new Uint8Array([1, 2, 3]);
@@ -19,6 +20,36 @@ assert.equal(EXAMPLE_SD_TEXT.split('Lorem ipsum').length - 1, 10);
 assert.equal(formatBytes(512), '512 B');
 assert.equal(formatBytes(64 * 1024 * 1024), '64.0 MiB');
 
+const fatPayload = new TextEncoder().encode('A'.repeat(700));
+const sdImage = buildFat32Image([
+  { path: 'Test/example.txt', bytes: fatPayload },
+  { path: 'Books/A long filename.txt', bytes: new TextEncoder().encode('book') },
+]);
+const sdView = new DataView(sdImage.buffer);
+assert.equal(sdImage.length, FAT32_IMAGE_SIZE);
+assert.equal(sdView.getUint16(11, true), 512);
+assert.equal(sdImage[13], 1);
+assert.equal(sdView.getUint32(32, true), FAT32_IMAGE_SIZE / 512);
+assert.equal(sdView.getUint32(44, true), 2);
+assert.deepEqual([...sdImage.slice(510, 512)], [0x55, 0xaa]);
+const fatSectors = sdView.getUint32(36, true);
+const fatOffset = 32 * 512;
+const dataOffset = (32 + 2 * fatSectors) * 512;
+const root = sdImage.subarray(dataOffset, dataOffset + 512);
+const testEntryOffset = Array.from({ length: 16 }, (_, i) => i * 32)
+  .find(offset => root[offset + 11] !== 0x0f && new TextDecoder().decode(root.subarray(offset, offset + 8)).trim() === 'TEST');
+assert.notEqual(testEntryOffset, undefined);
+const testCluster = new DataView(root.buffer, root.byteOffset + testEntryOffset, 32).getUint16(26, true);
+const testDirectory = sdImage.subarray(dataOffset + (testCluster - 2) * 512, dataOffset + (testCluster - 1) * 512);
+const fileOffset = Array.from({ length: 16 }, (_, i) => i * 32)
+  .find(offset => new TextDecoder().decode(testDirectory.subarray(offset, offset + 8)).trim() === 'EXAMPLE');
+const fileEntry = new DataView(testDirectory.buffer, testDirectory.byteOffset + fileOffset, 32);
+const fileCluster = fileEntry.getUint16(26, true);
+assert.equal(fileEntry.getUint32(28, true), fatPayload.length);
+assert.equal(sdView.getUint32(fatOffset + fileCluster * 4, true), fileCluster + 1);
+assert.equal(sdView.getUint32(fatOffset + (fileCluster + 1) * 4, true), 0x0fffffff);
+assert.deepEqual(sdImage.slice(dataOffset + (fileCluster - 2) * 512, dataOffset + (fileCluster - 2) * 512 + fatPayload.length), fatPayload);
+
 const calls = [];
 const module = {
   _xteink_wasm_set_adc: (...args) => calls.push(['adc', ...args]),