proto: freeze the wire contract at 1.0.0

Schemas for the whole v1 surface: 38 methods, 9 events, 25 named types and the
JSON-RPC envelope, with x-privileged / x-transports / x-deadlineMs / x-errors
annotations that both generators emit as data rather than prose.

Four generators over one IR (contracts/codegen/schema_ir.py), so the C++ structs,
the TypeScript types and the OpenRPC document cannot disagree about what the
contract says:

  gen_cpp.py             -> core/generated/velox_proto.{hpp,cpp}
  gen_ts.py              -> extension/src/shared/protocol/
  gen_openrpc.py         -> contracts/openrpc.json
  gen_cpp_conformance.py -> tests/conformance/cpp/fixture_dispatcher.hpp

Inbound parsing never throws: parse<T>() returns std::expected<T, ParseError> and
nlohmann's throwing ADL from_json is deliberately not emitted. Schema constraints
(minimum, maxLength, pattern, ...) become real runtime checks in both languages —
the daemon does not trust the extension and the extension does not trust the
daemon.

59 golden fixtures: a success case per method, 12 error cases, 9 events. Replayed
by tests/conformance/ against both the generated C++ and a live server over both
transports. tools/mockd serves the same fixtures with unhappy-path flags so the
GUI and EXT lanes never wait for veloxd.

run.sh also proves capture.offer fails open: with a daemon answering slower than
750 ms the client gives up and lets Firefox take the download.

core/generated/ is libveloxproto, a separate target from libveloxcore, which
still never sees JSON — see docs/adr/0009.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_012fgjnqFCS5h5L7gZTZo3rV

contracts/fixtures/download.add.json

@@ -0,0 +1,31 @@
+{
+  "name": "download.add \u2014 start an ISO now, into the Programs category",
+  "description": "The ordinary add path. saveDir is canonicalized and checked against the allowed roots before anything is written.",
+  "request": {
+    "jsonrpc": "2.0",
+    "id": 11,
+    "method": "download.add",
+    "params": {
+      "url": "https://releases.ubuntu.com/26.04/ubuntu-26.04-desktop-amd64.iso",
+      "filename": "ubuntu-26.04-desktop-amd64.iso",
+      "saveDir": "/home/sami/Downloads/Programs",
+      "categoryId": "programs",
+      "segments": 8,
+      "startMode": "now"
+    }
+  },
+  "response": {
+    "jsonrpc": "2.0",
+    "id": 11,
+    "result": {
+      "taskId": "$uuid",
+      "state": "connecting",
+      "duplicate": null
+    }
+  },
+  "assertions": [
+    "the .veloxpart file is created sparse and preallocated at the final size",
+    "saveDir resolves inside saveTo.allowedRoots, or the call fails -32011 having written nothing",
+    "event.task.added is emitted to every subscriber before this reply is sent"
+  ]
+}