docs: document the Android NEON/JNI kernel backend; fix a real Panama-on-Android bug

[michalharakal]

Summary

skainet-backend-jni-cpu has shipped since 0.39.0 (real, benchmarked
NEON acceleration for Android) but had essentially zero Antora
coverage — only a bare native-jni cell in the generated kernel matrix,
no prose anywhere, and the architecture reference's own ADR read as if
JNI had been rejected in favor of FFM, when the real story is
FFM-where-available / JNI-where-ART-has-no-FFM.

• New explanation/perf/android-neon-jni-kernels.adoc: why Android
  needs its own provider, the two .so tiers (/proc/cpuinfo-gated
  armv8-a vs armv8.2+dotprod), the JNI-done-carefully details
  (GetPrimitiveArrayCritical, eager library load, no-underscore method
  names), the smoke-test availability probe, and the real benchmark
  numbers (~24 tok/s vs ~3.8 tok/s scalar, Pixel 8a).
• architecture.adoc: added skainet-backend-jni-cpu to the module
  table and a new "Native (JNI) provider" note box; converted the ASCII
  kernel-SPI diagram to Mermaid and added the JNI provider box;
  corrected the ADR row that read as "JNI rejected" to state what was
  actually decided (FFM over JNI for the JVM provider, not a blanket
  rejection).

A real bug, found by cross-checking claims against generated output

While writing this I checked every claim against the machine-generated
kernel-support-matrix.adoc rather than just prose, and found
KernelSupportMatrixTest hardcoded panama-vector as available on
{JVM, Android} — but jdk.incubator.vector cannot run on ART at all;
PlatformCpuOpsFactory.android only ever registers ServiceLoader-discovered
providers + the scalar floor. Android's Float32/BFloat16 cells were
wrongly showing panama-vector; fixed to scalar. Same investigation
also caught an unrelated inaccuracy repeated in two places: Q6_K has no
FFM kernel, so its JVM cell is panama-vector, not native-ffm — fixed
in the architecture.adoc note box and the eager-backends mindmap.
Regenerated kernel-support-matrix.adoc via ./gradlew generateKernelMatrix.

Also updated

docs/eager-execution-backends-and-kernels.md (the hand-authored
mindmap referenced from the README, not part of Antora): added Native JNI and Native cinterop nodes it was missing entirely, fixed the same
Panama-on-Android inaccuracy, fixed a stale "#708 not done" status
(Q5_1/Q5_0 FFM shipped in 0.39.1), and split the kernel × provider table
by platform instead of conflating JVM and Android under one column.

Test plan

• ./gradlew :skainet-backends:skainet-backend-native-cpu:jvmTest --tests '*KernelSupportMatrixTest*' — passes with the corrected tier declaration
• Full local Antora build (docker build docs/.docker + antora-playbook.yml) — new page renders, Mermaid diagram renders as inline SVG, nav entry resolves. (One pre-existing, unrelated error: architecture.adoc's image::SKaiNET-compiler.svg[] — the file is .png not .svg on develop already, not introduced by this PR.)

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