Serialize streamed Bake exceptions into fallback payloads

The unchanged pinned Bun test/bake/dev/react-response.test.ts reaches 9 pass / 2 fail under Home. Both failures are streaming error cases whose HTML contains __bunfallback, but problems.exceptions is empty because Home still stubs ZigException.addToErrorList. Port Bun’s api.JsException projection and ZigStackTrace.toAPI path so streamed SSR exceptions retain their name, message, and stack in the Pechy fallback payload.\n\nParent: #555\nUmbrella: #66\nPinned Bun SHA: 4982b91e3702094330f3be3883354c52b8c01323

Implemented and pushed in 1f31d1836.

Home now ports Bun’s ZigException.addToErrorList, ZigStackTrace.toAPI, ZigStackFrame.toAPI, and source-line projection path. Streamed SSR errors therefore retain their name, message, runtime type, error code, frames, and source lines in the Pechy fallback payload instead of serializing an empty problems.exceptions array.

Verification:

• unchanged pinned test/bake/dev/react-response.test.ts: 11 pass, 0 fail
• focused native projection regression: pass
• home_rt with JSC: 1,815 pass, 19 skip, 0 fail
• test-bake-development: pass
• JSC-enabled debug build: pass
• JSC-disabled debug build: pass
• Pickier on all three changed files: pass
• temporary corpus overlays removed before commit