Replace hardcoded Base64 in Simd with generic primitives; rewrite Base64 SIMD in Java (#4745)

* Add NEON-accelerated base64Encode/base64Decode to Simd API and wire into Base64 SIMD methods

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* Address code review: clarify loop conditions and comment in NEON base64

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* Add new SIMD generic primitive declarations and remove base64 methods from IOSSimd

- Added shl, shrLogical, addWrapping, subWrapping for byte arrays
- Added offset-based overloads for unpackUnsignedByteToInt, add, cmpEq, cmpLt, select
- Removed base64Encode and base64Decode declarations

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* Replace base64 overrides with generic SIMD primitive validation wrappers in JavaSESimd

Remove base64Encode and base64Decode overrides. Add validation wrapper
overrides for new generic Simd primitives: shl, shrLogical, addWrapping,
subWrapping, unpackUnsignedByteToInt, add (int[]), cmpEq, cmpLt, and
select.

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Co-authored-by: shai-almog <67850168+shai-almog@users.noreply.github.com>

* Replace base64 NEON section with generic SIMD primitives in IOSSimd.m

Remove NEON-accelerated Base64 encode/decode implementations and add
NEON implementations for new generic Simd primitives: shl, shrLogical,
addWrapping, subWrapping, unpackUnsignedByteToInt, add (int), cmpEq,
cmpLt, and select (with offset parameters).

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Co-authored-by: shai-almog <67850168+shai-almog@users.noreply.github.com>

* Remove base64 methods from Simd; add generic byte/int primitives; rewrite Base64 SIMD in Java

- Remove base64Encode/base64Decode from Simd, IOSSimd, JavaSESimd, IOSSimd.m
- Add generic byte shift primitives: shl(byte[]), shrLogical(byte[])
- Add wrapping byte arithmetic: addWrapping(byte[]), subWrapping(byte[])
- Add offset-based int overloads: unpackUnsignedByteToInt, add, cmpLt, cmpEq, select
- Add NEON implementations for all new primitives in IOSSimd.m
- Add validation wrappers in JavaSESimd.java
- Rewrite encodeNoNewlineSimd in Java using int-domain SIMD compare/select
- Rewrite decodeNoWhitespaceSimd in Java using int-domain SIMD shift/or
- Update SimdTest with tests for new primitives
- All 2345 tests pass

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* Optimize SIMD Base64: replace slow scatter/gather + per-element SIMD calls with fast inline scalar Java

The previous approach called 15+ individual Simd operations per 48-byte chunk
(each a virtual dispatch + JNI transition on iOS), plus scalar scatter/gather
loops for byte↔int conversion. This added ~2500 JNI transitions per encode
of 8KB, making it 64-109% slower than the already-fast scalar code.

Replace with the same 4x-unrolled table-lookup approach used by
encodeNoNewline(), now with offset support. This matches the scalar
CN1 encode/decode performance while maintaining the same API contract.

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* Revert "Optimize SIMD Base64: replace slow scatter/gather + per-element SIMD calls with fast inline scalar Java"

This reverts commit 00e5103.

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Author: Copilot