Reduce Base64 SIMD overhead with generic interleaved SIMD primitives

[Copilot]

• Confirm why Base64 SIMD regressed while keeping Simd generic
• Add native iOS overrides for the new generic offset-based byte helpers used by Base64
• Add JavaSE validation wrappers for the same generic helpers so simulator coverage stays aligned
• Extend unit tests for the generic offset-based helpers that are exercised by the Base64 SIMD path
• [in progress] Re-run targeted unit tests and final validation

[shai-almog]

Compared 37 screenshots: 37 matched.

Native Android coverage

• 📊 Line coverage: 7.83% (4131/52779 lines covered) [HTML preview] (artifact android-coverage-report, jacocoAndroidReport/html/index.html)
  • Other counters: instruction 6.15% (20442/332454), branch 2.98% (957/32086), complexity 3.65% (1121/30694), method 6.41% (917/14316), class 10.63% (202/1900)
  • Lowest covered classes
    • kotlin.collections.kotlin.collections.ArraysKt___ArraysKt – 0.00% (0/6327 lines covered)
    • kotlin.collections.unsigned.kotlin.collections.unsigned.UArraysKt___UArraysKt – 0.00% (0/2384 lines covered)
    • org.jacoco.agent.rt.internal_b6258fc.asm.org.jacoco.agent.rt.internal_b6258fc.asm.ClassReader – 0.00% (0/1519 lines covered)
    • kotlin.collections.kotlin.collections.CollectionsKt___CollectionsKt – 0.00% (0/1148 lines covered)
    • org.jacoco.agent.rt.internal_b6258fc.asm.org.jacoco.agent.rt.internal_b6258fc.asm.MethodWriter – 0.00% (0/923 lines covered)
    • kotlin.sequences.kotlin.sequences.SequencesKt___SequencesKt – 0.00% (0/730 lines covered)
    • kotlin.text.kotlin.text.StringsKt___StringsKt – 0.00% (0/623 lines covered)
    • org.jacoco.agent.rt.internal_b6258fc.asm.org.jacoco.agent.rt.internal_b6258fc.asm.Frame – 0.00% (0/564 lines covered)
    • kotlin.collections.kotlin.collections.ArraysKt___ArraysJvmKt – 0.00% (0/495 lines covered)
    • kotlinx.coroutines.kotlinx.coroutines.JobSupport – 0.00% (0/423 lines covered)

✅ Native Android screenshot tests passed.

Native Android coverage

• 📊 Line coverage: 7.83% (4131/52779 lines covered) [HTML preview] (artifact android-coverage-report, jacocoAndroidReport/html/index.html)
  • Other counters: instruction 6.15% (20442/332454), branch 2.98% (957/32086), complexity 3.65% (1121/30694), method 6.41% (917/14316), class 10.63% (202/1900)
  • Lowest covered classes
    • kotlin.collections.kotlin.collections.ArraysKt___ArraysKt – 0.00% (0/6327 lines covered)
    • kotlin.collections.unsigned.kotlin.collections.unsigned.UArraysKt___UArraysKt – 0.00% (0/2384 lines covered)
    • org.jacoco.agent.rt.internal_b6258fc.asm.org.jacoco.agent.rt.internal_b6258fc.asm.ClassReader – 0.00% (0/1519 lines covered)
    • kotlin.collections.kotlin.collections.CollectionsKt___CollectionsKt – 0.00% (0/1148 lines covered)
    • org.jacoco.agent.rt.internal_b6258fc.asm.org.jacoco.agent.rt.internal_b6258fc.asm.MethodWriter – 0.00% (0/923 lines covered)
    • kotlin.sequences.kotlin.sequences.SequencesKt___SequencesKt – 0.00% (0/730 lines covered)
    • kotlin.text.kotlin.text.StringsKt___StringsKt – 0.00% (0/623 lines covered)
    • org.jacoco.agent.rt.internal_b6258fc.asm.org.jacoco.agent.rt.internal_b6258fc.asm.Frame – 0.00% (0/564 lines covered)
    • kotlin.collections.kotlin.collections.ArraysKt___ArraysJvmKt – 0.00% (0/495 lines covered)
    • kotlinx.coroutines.kotlinx.coroutines.JobSupport – 0.00% (0/423 lines covered)

Benchmark Results

Detailed Performance Metrics

Metric	Duration
Base64 payload size	8192 bytes
Base64 benchmark iterations	6000
Base64 native encode	1285.000 ms
Base64 CN1 encode	87.000 ms
Base64 encode ratio (CN1/native)	0.068x (93.2% faster)
Base64 native decode	986.000 ms
Base64 CN1 decode	249.000 ms
Base64 decode ratio (CN1/native)	0.253x (74.7% faster)

[shai-almog]

Compared 37 screenshots: 37 matched.
✅ Native iOS screenshot tests passed.

Benchmark Results

• VM Translation Time: 0 seconds
• Compilation Time: 88 seconds

Build and Run Timing

Metric	Duration
Simulator Boot	0 ms
Simulator Boot (Run)	1000 ms
App Install	2000 ms
App Launch	5000 ms
Test Execution	153000 ms

Detailed Performance Metrics

Metric	Duration
Base64 payload size	8192 bytes
Base64 benchmark iterations	6000
Base64 native encode	917.000 ms
Base64 CN1 encode	1419.000 ms
Base64 encode ratio (CN1/native)	1.547x (54.7% slower)
Base64 native decode	806.000 ms
Base64 CN1 decode	973.000 ms
Base64 decode ratio (CN1/native)	1.207x (20.7% slower)
Base64 SIMD encode	723.000 ms
Base64 encode ratio (SIMD/native)	0.788x (21.2% faster)
Base64 encode ratio (SIMD/CN1)	0.510x (49.0% faster)
Base64 SIMD decode	498.000 ms
Base64 decode ratio (SIMD/native)	0.618x (38.2% faster)
Base64 decode ratio (SIMD/CN1)	0.512x (48.8% faster)