feat: SIMD-0321 provide instruction-data VM address in r2

[7layermagik]

Summary

Implements SIMD-0321, which provides the VM address of the instruction data buffer in register r2 at program entrypoint. This allows BPF programs to locate their instruction data within the serialized input region without scanning.

Changes

• Feature gate: Added ProvideInstructionDataOffsetInVmR2 (5xXZc66h4UdB6Yq7FzdBxBiRAFMMScMLwHxk2QZDaNZL) to pkg/features/gates.go
• Serialization: serializeParametersAligned and serializeParametersUnaligned now return the byte offset of the instruction data within the serialized input buffer (offset past the 8-byte length prefix). Both aligned and unaligned paths compute this identically.
• VM plumbing: Added InputDataVaddr field to VMOpts and Interpreter structs. When the feature is active, InputDataVaddr is set to VaddrInput + instrDataOffset, giving the absolute VM address of the instruction data bytes.
• Entrypoint: r2 is initialized to InputDataVaddr at interpreter startup (alongside the existing r1 = VaddrInput). When the feature is inactive, r2 remains 0 (no behavioral change for existing programs).

Agave reference

• agave/programs/bpf_loader/src/lib.rs — create_vm sets r2 from instruction_data_offset
• agave/programs/bpf_loader/src/serialization.rs — serialize_parameters_aligned returns instruction_data_offset
• Feature gate: provide_instruction_data_offset_in_vm_r2

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