chore: remove native extension-specific code

[stephenh-axiom-xyz]

Resolves INT-6435. Documentation update will be part of a larger change.

Summary

Remove remaining code-level references to the Native extension, which was deleted in an earlier commit (4d54346f6). This cleans up leftover Native-specific APIs, types, and helper code that are no longer used now that the deferral extension handles the deferral address space directly.

1. Remove KvStore and hint-load-by-key

• Delete KvStore trait and its HashMap impl from Streams (crates/vm/src/arch/vm.rs)
• Remove kv_store field from Streams and StdIn, along with add_key_value helper (crates/sdk/src/stdin.rs)
• Delete Rv32HintLoadByKeySubEx phantom sub-executor and its encode/decode helpers (extensions/rv32im/circuit/src/extension/mod.rs)
• Remove HintLoadByKey variant from PhantomImm enum and Rv32Phantom enum (extensions/rv32im/guest/src/lib.rs, extensions/rv32im/transpiler/src/instructions.rs)
• Delete hint_load_by_key guest intrinsic (extensions/rv32im/guest/src/io.rs) and its re-export (crates/toolchain/openvm/src/io/mod.rs)
• Remove transpiler handling for PhantomImm::HintLoadByKey (extensions/rv32im/transpiler/src/lib.rs)
• Delete hint_load_by_key example program and its integration test (extensions/rv32im/tests/)

2. Remove native store-to-deferral-AS from rv32im loadstore

• Remove NATIVE_STOREW_FUNCT3, NATIVE_STOREW_FUNCT7 constants and store_to_native! macro from guest (extensions/rv32im/guest/src/lib.rs, extensions/rv32im/guest/src/io.rs)
• Delete store_u32_to_native wrapper (crates/toolchain/openvm/src/io/mod.rs)
• Remove transpiler handling for NATIVE_STOREW_FUNCT3 R-type instruction (extensions/rv32im/transpiler/src/lib.rs)
• Reject DEFERRAL_AS as an invalid address space in loadstore execution and trace generation, rather than handling it as a special case (extensions/rv32im/circuit/src/loadstore/execution.rs, extensions/rv32im/circuit/src/adapters/loadstore.rs)
• Remove is_native_store variable and all F-typed (field-element) store dispatch branches from the loadstore executor; stores are now always U8-typed (extensions/rv32im/circuit/src/loadstore/execution.rs)
• Simplify AOT: is_aot_supported now unconditionally returns true; delete is_aot_supported_impl which checked for deferral-AS stores (extensions/rv32im/circuit/src/loadstore/aot.rs)

3. Move deferral memory helpers from rv32im to deferral crate

• Delete extensions/rv32im/circuit/src/adapters/deferral.rs (199 lines) and its re-export
• Create extensions/deferral/circuit/src/adapters.rs with the subset of helpers still needed: memory_read_deferral, memory_write_deferral, timed_read_deferral, timed_write_deferral, tracing_read_deferral, tracing_write_deferral
• Functions only needed by the old Native extension (memory_read_or_imm_deferral, *_from_state, tracing_read_or_imm_deferral, tracing_write_deferral_inplace) are dropped
• Update import in extensions/deferral/circuit/src/call/trace.rs

4. Rename Native terminology in types and comments

• MemoryCellType::Native { size } -> MemoryCellType::F { size } and native32() -> field32() (crates/vm/src/arch/config.rs + all match sites)
• DEFAULT_NATIVE_BLOCK_SIZE -> DEFAULT_FIELD_BLOCK_SIZE
• Delete MemoryConfig::aggregation() (only used for native-extension testing) and VmChipTestBuilder::default_native()
• Update comments from "native address space" to "deferral address space" in AIR docs and assertions
• Update Poseidon2 chip module doc to remove "native kernel" language

5. Delete CUDA native adapter

• Delete crates/vm/cuda/include/system/native_adapter.cuh
• Remove native:: C++ namespace from crates/circuits/primitives/cuda/include/primitives/constants.h
• Update comment in extensions/rv32im/circuit/cuda/src/loadstore.cu

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Code Review: Remove Native Extension-Specific Code

This is a thorough and well-structured removal of dead code. The changes are consistent across Rust, CUDA, guest, and transpiler layers. A few items to consider:

Issues

1. Dead LoadStoreOp<F> impls (medium)

extensions/rv32im/circuit/src/loadstore/execution.rs:401-444 — The three impl<F: PrimeField32> LoadStoreOp<F> blocks for StoreWOp, StoreHOp, and StoreBOp are now dead code. The dispatch! macro (line 86-104) exclusively uses the U8 type parameter for all arms — no arm instantiates LoadStoreOp<F>. These impls were only needed when native stores wrote field elements (F-typed) to the deferral address space, which has been removed.

Fix this →

2. Stale prev_data comments (minor)

Now that DEFERRAL_AS is rejected as InvalidInstruction in pre_compute_impl (execution.rs:54), prev_data can no longer come from the deferral address space through the loadstore path. The comments were updated from "native" to "deferral" but the stated rationale no longer applies:

• extensions/rv32im/circuit/src/loadstore/core.rs:244: // Note: prev_data can be a field, so we need to use u32
• extensions/rv32im/circuit/cuda/src/loadstore.cu:28: same comment

Since stores are now always U8-typed, prev_data values will always fit in u8. The u32 representation may still be justified for uniformity with the run_write_data return type, but the "can be a field" explanation is wrong. Consider updating or removing these comments.

Fix this →

3. Stale comment in common/mod.rs (nit)

extensions/rv32im/circuit/src/common/mod.rs:160: The comment says "If we ignore the deferral address space..." — but now that loadstore rejects DEFERRAL_AS, there's nothing to "ignore." The comment could be simplified to just state the fact directly.

4. AOT assert_ne! vs returning error (nit)

In extensions/rv32im/circuit/src/loadstore/aot.rs:115, the code returns Err(AotError::InvalidInstruction) for RV32_IMM_AS, which is consistent. But the DEFERRAL_AS check on the same line uses the same pattern — good. However, there's no unused_parameter lint for the inst param in is_aot_supported (line 21-23). The parameter is now completely ignored since the function unconditionally returns true. A leading underscore (_inst) would be more idiomatic and suppress any future lints.

Looks Good

• KvStore removal is thorough — trait, impl, fields, helpers, phantom sub-executor, encode/decode, guest intrinsic, transpiler handling, example program, and test are all removed.

• Deferral helpers migration to extensions/deferral/circuit/src/adapters.rs correctly retains only the subset still used. The dropped functions were Native-extension-only. Import update in call/trace.rs is correct.

• Native → F rename is applied consistently: MemoryCellType::Native → F, native32() → field32(), DEFAULT_NATIVE_BLOCK_SIZE → DEFAULT_FIELD_BLOCK_SIZE.

• Loadstore simplification — dispatch macro reduced from 18 arms to 12 by removing the is_native_store dimension. Adding debug_assert_ne!(e, DEFERRAL_AS) at the adapter level provides defense-in-depth.

• CUDA cleanup — native_adapter.cuh deleted, native:: namespace removed from constants.h.

• AOT simplification — is_aot_supported now unconditionally returns true, removing the dead is_aot_supported_impl.

• New adapters.rs in the deferral crate is well-structured with correct safety comments for the unsafe memory operations.