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/acadrust-dwg/acadrust_dwg.d.ts

@@ -3,15 +3,30 @@
 
 export function parse_dwg(bytes: Uint8Array): any;
 
+/**
+ * The same parseResult as [`parse_dwg`], returned as a JSON string for the
+ * caller to hand to `JSON.parse`. Prefer this on anything but small drawings.
+ *
+ * Materializing the whole result as a `serde_json::Value` is what makes big
+ * files unopenable. The tree costs ~150× the DWG on the heap (a 19 MB drawing
+ * peaks near 2 GB), and the wasm allocator's per-allocation cost rises with the
+ * live heap, so conversion goes quadratic — measured 352 s in wasm for work
+ * that takes 2.5 s natively. Converting one entity at a time and appending its
+ * JSON text keeps the live heap flat and the cost linear.
+ */
+export function parse_dwg_json(bytes: Uint8Array): string;
+
 export type InitInput = RequestInfo | URL | Response | BufferSource | WebAssembly.Module;
 
 export interface InitOutput {
     readonly memory: WebAssembly.Memory;
     readonly parse_dwg: (a: number, b: number) => [number, number, number];
+    readonly parse_dwg_json: (a: number, b: number) => [number, number, number, number];
     readonly __wbindgen_malloc: (a: number, b: number) => number;
     readonly __wbindgen_realloc: (a: number, b: number, c: number, d: number) => number;
     readonly __wbindgen_externrefs: WebAssembly.Table;
     readonly __externref_table_dealloc: (a: number) => void;
+    readonly __wbindgen_free: (a: number, b: number, c: number) => void;
     readonly __wbindgen_start: () => void;
 }