Build one component instead of every one with a command

Pressing Update on one client of a multi-client project ran every
component's build to get the one that was actually asked for -- cheap
while a project had one APK, expensive the moment it had two and one of
them was slow (an ARM cross-compile, say). /prepare and /build now take
?component= to restrict a run to one named component; absent still means
the whole project, which is what Pull & Build, the project-row Rebuild,
and every single-component project keep doing. There is deliberately no
component-scoped Rebuild -- forcing one component's build without
touching the rest happens by pressing Update on it, which now runs
/prepare scoped to that component.

Verified against a live server driving a scratch two-Apk project:
building one component leaves the other's marker untouched, an unknown
?component= answers 404 naming the project and the component, and naming
none still builds both.

That verification surfaced a second, sharper bug the scoping change had
not caused but did make newly visible: component_is_stale's "never built
at all" check still asked find_apks of the whole project root, left
behind when per-component discovery (c4e19c2) moved everywhere else to
each component's own directory. A project with two Apk components has
one's output sitting under the root-anchored patterns too, so the moment
either component had ever been built, the whole project read as
"something is built here" -- and the other, never built, silently stopped
being offered its own first build. /prepare saw a component with a
command and no output and declared it current. Fixed by scoping the same
check to the component's own directory, guarded to Apk components only:
a Server never has an APK to find under its directory by definition, and
asking would have reported every server "never built" forever, which
broke two existing tests before the guard was added. Component::dir is
now the one definition of what a component's directory is, used by the
build command, the staleness check, and discovery alike.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

AGENTS.md

@@ -467,6 +467,42 @@ mutable at runtime from the phone.
   builds a component has, a reused name is not the same APK, and handing
   it the old one's package would be wrong until that component happened
   to be downloaded.
+- **`/prepare` and `/build` take `?component=` to build one component
+  instead of every one with a command.** Without it, pressing Update on
+  one client of a multi-client project ran every component's build to
+  get the one that was actually asked for -- fine when a project had one
+  APK, expensive the moment it had two and one of them was slow.
+  `named_component` in `routes.rs` is the one place a name from the phone
+  is checked against the project's own components, so `/prepare` and
+  `/build` cannot disagree about what an unknown name means, and
+  `BuildState::{trigger_if_needed,build_now,run_build,is_stale}` all take
+  the same `Option<&str>` -- `None` still means the whole project, which
+  is what `Pull & Build`, the project-row `Rebuild`, and every project
+  with a single component keep doing. There is deliberately no
+  component-scoped Rebuild: forcing one component's build without
+  touching the rest happens by pressing Update on it, which runs
+  `/prepare` scoped to that component.
+  **Where `component.dir()` belongs is now one definition**
+  (`Component::dir` in `config.rs`), because a second one very nearly
+  shipped a real bug: `component_is_stale`'s "never built at all" check
+  still asked `find_apks` of the *project root* after per-component
+  discovery had already moved everywhere else to `component.dir()`. A
+  project with two `Apk` components has one's output sitting under the
+  root-anchored patterns too -- `*/build/outputs/apk/*/*.apk` matches any
+  one-level subdirectory, regardless of which component put it there --
+  so the moment *either* component had ever been built, the whole
+  project read as "something is built here," and the *other* component,
+  never built, silently stopped being offered its own first build:
+  `prepare` saw a component with a command and no output and declared it
+  current. Caught by testing the actual behaviour of a two-APK project
+  rather than trusting that scoping the build implied scoping the
+  staleness check that decides whether to run it -- they are two
+  different reads of "which directory is this component's," and only one
+  of them had been moved.
+  The same check is deliberately *not* asked of a `Server`: a service
+  never has an APK to find under its own directory by definition, so
+  asking would report every server "never built" forever. Guarded on
+  `matches!(component, Component::Apk { .. })` for that reason.
 - **The self entry's project is the working directory itself**, so this
   server has to be started from the root of its own checkout -- which is
   where everything else here is driven from, and what the service unit