fix(viewer2d): sync dwg-wasm fix for large-DWG load and hatch arc artifacts

Mirrors hmwebviewer's rust/dwg-wasm change into the sample: the wasm
artifact plus the parser wrapper, which now goes through parse_dwg_json
and JSON.parse instead of the JsValue-returning parse_dwg.

Building the whole result as a serde_json::Value tree pushed the live
wasm heap near 2.9GB for a 19MB drawing, and the allocator's cost grows
with that heap, so parsing went quadratic — 354s, with the tab frozen
throughout. Streaming one entity at a time keeps it linear: 2.5s.

The same build also fixes three bugs that drew stray 4km grey shapes
where two CF-PATT SOLID hatches should be. Max hatch span 4655 -> 110.

Adds two docs recording both investigations, in the format of the
existing ones: measurements, the hypotheses that were ruled out, the
fixes, regression checks, and the MPL-2.0 position (acadrust is
unmodified, so no new source-disclosure obligation arises).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/viewer2d/acadrustParser.ts

@@ -2,7 +2,7 @@
  * acadrust DWG parser wrapper — loads the wasm built from rust/dwg-wasm
  * (wasm-pack --target web, committed under ./acadrust-dwg).
  */
-import init, { parse_dwg } from './acadrust-dwg/acadrust_dwg.js';
+import init, { parse_dwg_json } from './acadrust-dwg/acadrust_dwg.js';
 import wasmUrl from './acadrust-dwg/acadrust_dwg_bg.wasm?url';
 import type { CadParseResult } from './dwgParser';
 
@@ -14,7 +14,15 @@ export function initAcadrustParser(): Promise<void> {
   return ready;
 }
 
-/** Parse a DWG buffer. Requires initAcadrustParser() first. */
+/**
+ * Parse a DWG buffer. Requires initAcadrustParser() first.
+ *
+ * Goes through `parse_dwg_json` + `JSON.parse` rather than the `parse_dwg`
+ * export that hands back a JsValue. The latter keeps the entire result live on
+ * the wasm heap as it builds, and the allocator's cost grows with that heap —
+ * a 19 MB drawing took ~354 s and froze the tab. Streaming the result out as
+ * text keeps it linear: the same file now lands in ~2.5 s.
+ */
 export function parseDwgAcadrust(bytes: Uint8Array): CadParseResult {
-  return parse_dwg(bytes) as CadParseResult;
+  return JSON.parse(parse_dwg_json(bytes)) as CadParseResult;
 }