[Lane 3 · android] Precision/Recall/F1 — Android target parity check (enable host tests for skainet-backend-cpu) #1230

Sub-issue of #1222 (Precision, Recall and F1Score metrics).

Lane: 3 · Platform verification · Android
Skill needed: Android build tooling — running a Kotlin Multiplatform module's tests on this target. No tensor-internals or metrics knowledge required. The code under test is pure commonMain Kotlin over Double counters; this is a smoke check, not new implementation.
Size: s (~1–2 h — see Notes: host tests are not enabled for this module yet, so the first step is a small build change)
Blocked by: #1229 (F1Score merged to develop), or check out its branch early.

What to do

1. Check whether skainet-backends/skainet-backend-cpu runs commonTest on the Android
   target at all. As of today it does not: ./gradlew :skainet-backends:skainet-backend-cpu:tasks --all
   warns "The commonTest source directory exists, but android host tests are not
   enabled. To enable android host tests, add withHostTest {} to your android target
   configuration" and lists no Android test task.
2. Add withHostTest {} to the module's Android target configuration (the
   com.android.kotlin.multiplatform.library plugin, applied via sk.ainet.multiplatform
   / libs.plugins.androidMultiplatformLibrary — check whether it belongs in the
   convention plugin under build-logic/convention/ so every module gets it, and say
   which you chose in the PR).
3. Run the resulting host-test task (something like
   ./gradlew :skainet-backends:skainet-backend-cpu:testAndroidHostTest — take the name
   from the task list) and, if you have a device or emulator, the instrumented variant too.

Then, in every case:

• Confirm the metric tests (sk.ainet.exec.metrics.*) pass and the values match the JVM run
  bit-for-bit — pure Long counting and Double division, so any drift is itself a finding.
• Report on [Feature]: Precision, Recall and F1Score metrics #1222: target, toolchain versions, pass/fail, wall-clock, anything
  target-specific you hit (an expect/actual gap, a disabled test source set, a task
  that doesn't exist).

Acceptance

• Result posted on [Feature]: Precision, Recall and F1Score metrics #1222 with the exact Gradle task(s) run
• If something is wrong, a separate bug issue opened and linked — don't block the
  parent on investigating it here

Notes

Enabling host tests may surface other failing tests in commonTest that have never run
on Android before. If so, that is a real finding: report the list on the parent and open
one bug for it; do not try to fix them in this PR. The kmp / Android conventions live
in build-logic/convention/ — see Explanation → Android NEON kernels via JNI for how
the Android target is wired.