feat(wasm): port Ghostty's wasm32 build

Ports main_wasm.zig as crates/ghostty-wasm (cdylib exporting the Zig
names: atlas_*, face_*, deferred_face_*, shaper_*, config_*), the
web_canvas font face and shaper (backend selected on wasm32), os/wasm
(logging, target, JS interop) and config/Wasm.zig.

C-library dependencies (freetype, harfbuzz, fontconfig, onig, gettext,
D-Bus, rustix) are target-gated. Like Zig's wasm build, link regex
search (StringMap::search_iterator) is unavailable without Oniguruma;
Collection/SharedGrid/harfbuzz are compiled out on wasm32. ghostty-os
switches from forbid to deny unsafe_code for the JS import blocks.

The JS import ABI is currently a documented handle-based interface with
a reference host (js/ghostty_host.js); aligning it with zig-js's exact
import names follows separately.

Test Plan:
- cargo build --target wasm32-unknown-unknown -p ghostty-font \
  -p ghostty-config -p ghostty-terminal -p ghostty-vt -p ghostty-wasm
- cargo test -p ghostty-wasm (node runs the module against a fake canvas)

crates/ghostty-wasm/tests/node_test.js

@@ -0,0 +1,203 @@
+// Instantiates ghostty_wasm.wasm with stub JS imports and exercises the
+// exported config / atlas / canvas face / shaper functions. Run by
+// tests/node.rs: `node node_test.js <path-to-wasm>`.
+const fs = require('fs');
+const assert = require('assert');
+const path = require('path');
+const { createHost } = require(path.join(__dirname, '..', 'js', 'ghostty_host.js'));
+
+// A fake canvas: every glyph is a 7px wide, 12px ascent, 4px descent box
+// (emoji are 14px wide and "colored"). measureText records the font in use.
+class FakeCanvas {
+  constructor() {
+    this.width = 300;
+    this.height = 150;
+    this.style = {};
+    this.calls = [];
+    const self = this;
+    this.ctx = {
+      font: '',
+      textBaseline: 'alphabetic',
+      fillStyle: '',
+      clearRect: (...a) => self.calls.push(['clearRect', ...a]),
+      fillRect: (...a) => self.calls.push(['fillRect', ...a]),
+      fillText: (t, x, y) => {
+        self.lastText = { t, x, y };
+        self.calls.push(['fillText', t, x, y]);
+      },
+      measureText: (t) => {
+        const n = [...t].length;
+        const emoji = /\p{Extended_Pictographic}/u.test(t);
+        const w = emoji ? 14 : 7 * n;
+        return {
+          width: w + 1,
+          actualBoundingBoxLeft: 0,
+          actualBoundingBoxRight: w,
+          actualBoundingBoxAscent: 12,
+          actualBoundingBoxDescent: t.includes('_') ? 4 : 0.5,
+          _t: t,
+        };
+      },
+      getImageData: (x, y, w, h) => {
+        const data = new Uint8ClampedArray(w * h * 4);
+        const emoji = /\p{Extended_Pictographic}/u.test(self.lastText ? self.lastText.t : '');
+        for (let i = 0; i < w * h; i++) {
+          if (emoji) {
+            data[i * 4] = 200; // red channel => color
+            data[i * 4 + 3] = 255;
+          } else {
+            data[i * 4 + 3] = (i % 2) * 255; // alpha only (black text)
+          }
+        }
+        return { data };
+      },
+      putImageData: (...a) => self.calls.push(['putImageData', ...a]),
+    };
+  }
+  getContext(kind) {
+    assert.strictEqual(kind, '2d');
+    return this.ctx;
+  }
+}
+class FakeImageData {
+  constructor(data, w, h) {
+    this.data = data;
+    this.width = w;
+    this.height = h;
+  }
+}
+const canvases = [];
+const fakeGlobal = {
+  document: {
+    createElement: (tag) => {
+      assert.strictEqual(tag, 'canvas');
+      const c = new FakeCanvas();
+      canvases.push(c);
+      return c;
+    },
+    getElementById: () => null,
+  },
+  ImageData: FakeImageData,
+};
+
+(async () => {
+  const logs = [];
+  const host = createHost({ global: fakeGlobal, log: (s) => logs.push(s) });
+  const bytes = fs.readFileSync(process.argv[2]);
+  const { instance } = await WebAssembly.instantiate(bytes, host.imports);
+  host.setInstance(instance);
+  const x = instance.exports;
+  x.ghostty_wasm_init();
+
+  // ---- config ----
+  {
+    const cfg = x.config_new();
+    assert.ok(cfg);
+    const s = host.writeString('font-size = 18\nwindow-padding-x = 7\nnot-a-key = 1\n');
+    x.config_load_string(cfg, s.ptr, s.len);
+    s.free();
+    assert.strictEqual(x.config_diagnostics_count(cfg), 1);
+    x.config_finalize(cfg);
+    const cap = 1 << 16;
+    const out = x.wasm_alloc(cap);
+    const n = x.config_format(cfg, out, cap);
+    assert.ok(n > 0 && n <= cap);
+    const text = host.readString(out, n);
+    x.wasm_free(out, cap);
+    assert.match(text, /font-size = 18/);
+    assert.match(text, /window-padding-x = 7/);
+    x.config_free(cfg);
+  }
+
+  // ---- atlas ----
+  const atlas = x.atlas_new(64, 0); // grayscale
+  assert.ok(atlas);
+  assert.strictEqual(x.atlas_size(atlas), 64);
+
+  // ---- face ----
+  const f = host.writeString('monospace');
+  const face = x.face_new(f.ptr, f.len, 12, 0);
+  assert.ok(face, 'face_new');
+  const m = x.wasm_alloc(12);
+  x.face_metrics(face, m);
+  const metrics = new Uint32Array(instance.exports.memory.buffer, m, 3).slice();
+  assert.deepStrictEqual([...metrics], [7, 16, 4]);
+  assert.ok(canvases[0].ctx.font === '12px monospace', canvases[0].ctx.font);
+
+  // Text presentation face supports ascii/latin text but not color emoji
+  assert.strictEqual((x.face_glyph_index(face, 0x41) >>> 0), 0x41);
+  assert.strictEqual((x.face_glyph_index(face, 0x4e2d) >>> 0), 0x4e2d);
+  assert.strictEqual((x.face_glyph_index(face, 0x1f600) >>> 0), 0xffffffff);
+
+  // Render a glyph into the atlas
+  const g = x.face_render_glyph(face, atlas, 0x41);
+  assert.ok(g, 'face_render_glyph');
+  const gv = new Uint32Array(instance.exports.memory.buffer, g, 6);
+  assert.strictEqual(gv[0], 8); // width: 7 + 1 border
+  assert.strictEqual(gv[1], 14); // height: ceil(12 + 0.5) + 1
+  x.glyph_free(g);
+  const sz = x.wasm_alloc(8);
+  const dp = x.atlas_data(atlas, sz);
+  const len = new Uint32Array(instance.exports.memory.buffer, sz, 1)[0];
+  assert.strictEqual(len, 64 * 64);
+  const atlasBytes = new Uint8Array(instance.exports.memory.buffer, dp, len);
+  assert.ok(atlasBytes.some((b) => b === 255), 'atlas received glyph alpha');
+
+  // Emoji face
+  const ef = host.writeString('sans-serif');
+  const eface = x.face_new(ef.ptr, ef.len, 14, 1);
+  assert.ok(eface);
+  assert.strictEqual((x.face_glyph_index(eface, 0x1f600) >>> 0), 0x1f600);
+  assert.strictEqual((x.face_glyph_index(eface, 0x4e2d) >>> 0), 0xffffffff);
+
+  // ---- shaper ----
+  const sh = x.shaper_new(16);
+  x.shaper_prepare(sh);
+  // 'e' + U+0301, then 'x'
+  x.shaper_add_codepoint(sh, 0x65, 0);
+  x.shaper_add_codepoint(sh, 0x301, 0);
+  x.shaper_add_codepoint(sh, 0x78, 1);
+  const outp = x.wasm_alloc(16 * 8);
+  const cells = x.shaper_shape(sh, face, outp, 16);
+  assert.strictEqual(cells, 2);
+  const ov = new Uint32Array(instance.exports.memory.buffer, outp, 4);
+  assert.deepStrictEqual([ov[0], ov[1] >= 0x110000, ov[2], ov[3]], [0, true, 1, 0x78]);
+  // Render the reserved grapheme glyph through the canvas
+  const gg = x.face_render_glyph(face, atlas, ov[1]);
+  assert.ok(gg);
+  x.glyph_free(gg);
+  assert.strictEqual(canvases[0].lastText.t, 'é');
+
+  // shaper_test prints through a terminal and logs the shaped runs
+  const st = host.writeString('hello é');
+  x.shaper_test(sh, face, st.ptr, st.len);
+  assert.ok(logs.some((l) => l.startsWith('info(')), logs.join('\n'));
+
+  // ---- deferred face ----
+  const d = x.deferred_face_new(f.ptr, f.len, 0);
+  assert.ok(d);
+  const before = canvases.length;
+  x.deferred_face_load(d, 12);
+  assert.strictEqual(canvases.length, before + 1);
+  x.deferred_face_free(d);
+
+  // ---- atlas debug canvas ----
+  const h = x.atlas_debug_canvas(atlas);
+  assert.ok(h > 0);
+  const dc = host.value(h);
+  assert.strictEqual(dc.width, 64);
+  assert.ok(dc.calls.some((c) => c[0] === 'putImageData'));
+
+  // ---- cleanup / handle leaks ----
+  x.shaper_free(sh);
+  x.face_free(face);
+  x.face_free(eface);
+  x.atlas_free(atlas);
+  f.free();
+  ef.free();
+  st.free();
+  console.log('ok');
+})().catch((e) => {
+  console.error(e);
+  process.exit(1);
+});