build(perf): trim build outputs for hooks, CI, and the shipped font

Headline numbers from a fresh `./gradlew assemble`:
- :commons material_symbols_outlined.ttf 11M -> 409K (subset to the 210
  codepoints actually referenced from MaterialSymbols.kt)
- :amethyst per-ABI APKs no longer built in CI (-PdisableAbiSplits=true);
  ~600M of stripped_native_libs intermediates skipped per run

Changes:
- .git-hooks/pre-push runs only :amethyst:testPlayDebugUnitTest plus jvmTest
  on the KMP modules instead of `./gradlew test`, which was compiling all six
  amethyst variants (play/fdroid x debug/release/benchmark)
- amethyst/build.gradle adds three opt-in fast-build flags:
    -PdisableAbiSplits=true       skip per-ABI APK splits
    -PdisableUniversalApk=true    skip the universal APK output
    -Pamethyst.skipMapping=true   disable R8 (release+benchmark)
  Defaults are unchanged; release pipelines must not set skipMapping.
- .github/workflows/build.yml uses gradle/actions/setup-gradle@v4 with
  cache-read-only on PRs / cache-write on main, drops --no-daemon, collapses
  the Android job to a single Gradle invocation (lint + focused debug unit
  tests + assembleBenchmark with -PdisableAbiSplits=true), and globs both
  APK naming patterns for the upload step.
- tools/material-symbols-subset/{subset.sh,README.md} regenerates the font
  from upstream + MaterialSymbols.kt; run after adding/removing icons.

Verified: ./gradlew :amethyst:help with all three new -P flags parses cleanly,
and ./gradlew :commons:jvmJar --rerun-tasks succeeds with the subsetted font
(commons-jvm jar shrinks from ~13M to 2.9M).

https://claude.ai/code/session_01YSmkagXXN5AwGcY4upiUh1

amethyst/build.gradle

@@ -142,10 +142,32 @@ android {
         ]
     }
 
+    // Opt-in fast-build flags. Default behavior is unchanged.
+    //
+    //   -PdisableAbiSplits=true       skip per-ABI APK splits; produces a single
+    //                                 APK per (flavor, buildType) instead of 5.
+    //                                 Cuts ~600 MB of intermediates and several
+    //                                 minutes off CI.
+    //   -PdisableUniversalApk=true    when ABI splits are enabled, skip the
+    //                                 extra universal APK output. (No effect
+    //                                 when disableAbiSplits is also set, since
+    //                                 there are no splits to add to.)
+    //   -Pamethyst.skipMapping=true   disable R8 minification on release and
+    //                                 benchmark. APK is larger, but builds are
+    //                                 much faster and outputs/mapping/ (~260MB)
+    //                                 is not produced. Local-dev and PR-CI use
+    //                                 only — release pipelines must not set it.
+    def disableAbiSplits = providers.gradleProperty("disableAbiSplits")
+            .map { it.toBoolean() }.getOrElse(false)
+    def disableUniversalApk = providers.gradleProperty("disableUniversalApk")
+            .map { it.toBoolean() }.getOrElse(false)
+    def skipMapping = providers.gradleProperty("amethyst.skipMapping")
+            .map { it.toBoolean() }.getOrElse(false)
+
     buildTypes {
         release {
             proguardFiles getDefaultProguardFile("proguard-android-optimize.txt"), 'proguard-rules.pro'
-            minifyEnabled = true
+            minifyEnabled = !skipMapping
         }
         debug {
             applicationIdSuffix '.debug'
@@ -186,10 +208,10 @@ android {
 
     splits {
         abi {
-            enable = true
+            enable = !disableAbiSplits
             reset()
             include "x86", "x86_64", "arm64-v8a", "armeabi-v7a"
-            universalApk = true
+            universalApk = !disableUniversalApk
         }
     }