Add desktop Gecko vertical probe

docs/architecture.md

@@ -47,6 +47,8 @@ is boring, and generated fixtures from AMPG open without manual proxy setup.
 Generated backend metadata lives in
 [generated browser strategy](generated/browser-strategy.md).
 
+The first implementation path is documented in [desktop vertical](desktop-vertical.md).
+
 ## Relationship To AMPG
 
 AMPG publishes protocol-specific site variants. AMPB opens those variants with the right

docs/browser-builds.md

@@ -35,6 +35,33 @@ The script creates:
 - `/tmp/ampb-browser-build/artifacts`
 - `/tmp/ampb-browser-build/logs`
 
+## Desktop Build Probe
+
+Desktop is the first browser proof. The probe uses the current Gecko checkout and writes
+its mozconfig, object directory, caches, and logs under `/tmp`.
+
+Print the desktop build entrypoint help:
+
+```sh
+sh tools/browser-desktop-build-probe.sh
+```
+
+Run the first desktop configure probe:
+
+```sh
+AMPB_DESKTOP_BUILD_PROBE_MODE=configure sh tools/browser-desktop-build-probe.sh
+```
+
+The probe writes:
+
+- `/tmp/ampb-browser-build/mozconfigs/desktop-debug.mozconfig`
+- `/tmp/ampb-browser-build/obj/gecko-desktop-debug`
+- `/tmp/ampb-browser-build/logs/gecko-desktop-mach-help.log`
+- `/tmp/ampb-browser-build/logs/gecko-desktop-mach-configure.log`
+
+Configure mode may install Mozilla build Python helpers such as `zstandard` into mach
+virtualenvs under `/tmp/ampb-browser-build/cache/mozbuild`.
+
 ## Source Plan
 
 Print the canonical source targets and paths:
@@ -77,7 +104,8 @@ AMPB_INCLUDE_LEGACY_FIREFOX_ANDROID=1 sh tools/browser-source-sync.sh
 
 ## Android Build Probe
 
-Run the first Android build-discovery command from the `/tmp` checkout while keeping
+Android remains a first-class target, but it follows the desktop proof. Run the first
+Android build-discovery command from the `/tmp` checkout while keeping
 Gradle, pip, Python bytecode, and Mozilla build state in `/tmp`:
 
 ```sh

docs/desktop-vertical.md

@@ -0,0 +1,28 @@
+# Desktop Vertical
+
+> Status: draft | Updated 2026-07-07 | Applies to: AMPB desktop builders
+
+The first AMPB browser proof is desktop-first with mobile constraints kept in view. The
+vertical is a bundled Gecko desktop runtime, AMPB-owned profile state, and a managed Tor
+runtime for `.onion` browsing. Android and iOS follow the same routing and transport
+contracts after the desktop path is proven.
+
+## Build Path
+
+Use the external build workspace:
+
+```sh
+sh tools/browser-build-workspace.sh
+sh tools/browser-source-sync.sh
+AMPB_DESKTOP_BUILD_PROBE_MODE=configure sh tools/browser-desktop-build-probe.sh
+```
+
+The desktop probe keeps Gecko source, object files, downloaded toolchains, Python
+virtualenvs, pip cache, and logs under `/tmp/ampb-browser-build`.
+
+## Transport Path
+
+The first transport target is Tor through an AMPB-owned Arti/Tor runtime. The desktop
+vertical is not complete until AMPB can start the runtime, verify the SOCKS endpoint, and
+launch the bundled browser profile without clearnet fallback. I2P and Reticulum use the
+same route, consent, state, and health-check model after Tor works.