scaffold: project structure, wire contract, roadmap and agent briefs

Lays out Velox Download Manager (IDM-class download manager for Ubuntu
26.04) as a monorepo ready for parallel lane development. No implementation
code by design.

- docs/: architecture, roadmap M0-M7, IDM-parity GUI spec, engine design,
  Firefox extension spec, risks/spikes, packaging
- contracts/: wire-contract skeleton (JSON Schema + fixture templates) —
  the single synchronization point between lanes
- docs/agents/: one brief per lane (PROTO, CORE, DAEMON, GUI, EXT, PKG/QA)
  with owned directories, build order and definition of done
- CLAUDE.md: rules of engagement — lane ownership, layering, non-negotiables
- CMake scaffolding with dev/tsan/release/ci presets

Two environment findings shape the design: Firefox here is the Mozilla snap
(native-messaging risk, so the extension carries a loopback-WebSocket
fallback), and Wayland forbids passive clipboard monitoring (so clipboard
capture is explicit-action-first).

Co-Authored-By: Claude Opus 5 <[email protected]>

contracts/fixtures/capture.offer.take.json

@@ -0,0 +1,40 @@
+{
+  "name": "capture.offer — attachment on a monitored type is taken",
+  "description": "Golden fixture. tests/conformance replays this against the real daemon AND the TS client. If either side drifts, this goes red before the lanes ever integrate.",
+  "request": {
+    "jsonrpc": "2.0",
+    "id": 42,
+    "method": "capture.offer",
+    "params": {
+      "url": "https://releases.ubuntu.com/26.04/ubuntu-26.04-desktop-amd64.iso",
+      "method": "GET",
+      "tabUrl": "https://releases.ubuntu.com/26.04/",
+      "headers": {
+        "User-Agent": "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:154.0) Gecko/20100101 Firefox/154.0",
+        "Referer": "https://releases.ubuntu.com/26.04/",
+        "Accept": "*/*"
+      },
+      "cookies": [],
+      "contentType": "application/octet-stream",
+      "contentLength": 6228541440,
+      "contentDisposition": "attachment; filename=\"ubuntu-26.04-desktop-amd64.iso\"",
+      "filename": "ubuntu-26.04-desktop-amd64.iso",
+      "origin": "moz-extension://11111111-2222-3333-4444-555555555555"
+    }
+  },
+  "response": {
+    "jsonrpc": "2.0",
+    "id": 42,
+    "result": {
+      "action": "take",
+      "taskId": "$uuid",
+      "reason": null
+    }
+  },
+  "assertions": [
+    "response arrives within 750 ms",
+    "a task exists afterwards with state in [queued, connecting, downloading]",
+    "the task's saveDir resolves to the 'Programs' category folder for .iso",
+    "the Referer and User-Agent from params.headers are replayed on the daemon's own request"
+  ]
+}