docs(deps): add a worked deps: example manifest, docs, and test

There was no committed emb.yaml with a deps: block. Add one and document
the schema.

- example/ivi-homescreen/emb.yaml: a self-describing package with a real
  deps: block (graphics build deps for fedora/ubuntu/macos) alongside a
  build: block.
- README: a 'Host dependencies (deps:)' section — the OS/distro schema,
  list-vs-map semantics, WhatProvides name resolution, the legacy
  configs/*.json schema, and an 'emb deps --packages example --dry-run'.
- test: load the committed example and assert deps resolve to the right
  packages per host (fedora/ubuntu/macos), excluding other hosts' names.

Signed-off-by: Joel Winarske <joel.winarske@gmail.com>

README.md

@@ -342,6 +342,35 @@ deps:                      # optional host packages, by OS / distro
   windows: [Kitware.CMake]
 ```
 
+#### Host dependencies (`deps:`)
+
+The `deps:` block declares host build packages, keyed by **OS**, then optionally
+by **distro id** on Linux:
+
+- A **list** directly under an OS (`macos: [...]`, `windows: [...]`, or
+  `linux: [...]`) applies to **any** distro on that OS.
+- A **map** under `linux` (`fedora: [...]`, `ubuntu: [...]`, …) selects by the
+  host's distro id (from `/etc/os-release`, what `emb doctor` prints).
+
+`emb deps` coalesces these across every selected manifest, filters to what's
+**missing** on the current host, and installs the union in one transaction
+(PackageKit on Linux, brew on macOS). Resolution is by package *name*, mapped to
+the backend's real package via `WhatProvides`, so e.g. `pkg-config` resolves
+even where the package is `pkgconf`.
+
+A complete worked manifest (ivi-homescreen graphics deps for fedora/ubuntu/macOS)
+is in [`example/ivi-homescreen/emb.yaml`](example/ivi-homescreen/emb.yaml).
+Inspect the resolved/missing plan for your host without installing:
+
+```sh
+emb deps --packages example --dry-run
+```
+
+> Legacy `configs/*.json` declare deps differently — as inline
+> `sudo … install` strings under `runtime.pre-requisites[arch][distro][version]`
+> — from which `emb` extracts the package names. Both schemas normalize to the
+> same per-host rule set.
+
 ---
 
 ### `emb cross`

example/ivi-homescreen/emb.yaml

@@ -0,0 +1,45 @@
+# A self-describing package manifest. `emb build .` builds its arch x mode
+# matrix; `emb deps --packages <parent-dir>` installs the host build deps below.
+# See ../README.md and the top-level README "Host dependencies (deps:)".
+id: ivi-homescreen
+type: app
+supported_archs: [arm64, x86_64]
+supported_host_types: [fedora, ubuntu, darwin]
+
+build:
+  app_path: .
+  archs: [arm64, x86_64]
+  modes: [release]
+  output: bundles
+
+# Host build dependencies, keyed by OS then (on Linux) distro id. A list
+# directly under an OS applies to any distro on that OS. `emb deps` coalesces
+# these across all discovered packages, filters to what's missing on the host,
+# and installs the union in one PackageKit (Linux) / brew (macOS) transaction.
+deps:
+  linux:
+    fedora:
+      - pkg-config
+      - mesa-libEGL-devel
+      - mesa-libGLES-devel
+      - libdrm-devel
+      - mesa-libgbm-devel
+      - libinput-devel
+      - libxkbcommon-devel
+      - wayland-devel
+      - wayland-protocols-devel
+      - systemd-devel
+    ubuntu:
+      - pkg-config
+      - libegl-dev
+      - libgles2-mesa-dev
+      - libdrm-dev
+      - libgbm-dev
+      - libinput-dev
+      - libxkbcommon-dev
+      - libwayland-dev
+      - wayland-protocols
+      - libudev-dev
+  macos:
+    - pkg-config
+    - wayland

test/src/manifest/manifest_loader_test.dart

@@ -196,4 +196,45 @@ emb:
       );
     });
   });
+
+  group('example/ivi-homescreen deps', () {
+    final dir = Directory(p.join('example', 'ivi-homescreen'));
+    const ubuntu = HostInfo(
+      os: HostOs.linux,
+      machineArch: 'x86_64',
+      archAliases: {'x86_64', 'x64', 'amd64'},
+      hostType: 'ubuntu',
+      versionId: '24.04',
+    );
+    const macos = HostInfo(
+      os: HostOs.macos,
+      machineArch: 'arm64',
+      archAliases: {'arm64'},
+      hostType: 'darwin',
+      versionId: '14',
+    );
+
+    test('the committed example manifest loads', () {
+      expect(dir.existsSync(), isTrue, reason: 'missing ${dir.path}');
+      final m = loader.loadPackageDir(dir);
+      expect(m, isNotNull);
+      expect(m!.id, 'ivi-homescreen');
+    });
+
+    test('deps resolve to the right packages per host', () {
+      final m = loader.loadPackageDir(dir)!;
+
+      final onFedora = m.deps.resolve(fedora);
+      expect(onFedora, containsAll(['pkg-config', 'mesa-libEGL-devel']));
+      expect(onFedora, isNot(contains('libegl-dev'))); // ubuntu-only name
+
+      final onUbuntu = m.deps.resolve(ubuntu);
+      expect(onUbuntu, containsAll(['pkg-config', 'libegl-dev']));
+      expect(onUbuntu, isNot(contains('mesa-libEGL-devel'))); // fedora-only
+
+      final onMac = m.deps.resolve(macos);
+      expect(onMac, containsAll(['pkg-config', 'wayland']));
+      expect(onMac, isNot(contains('libdrm-devel'))); // linux-only
+    });
+  });
 }