feat(arithmetization): sha2 precompile

[arijitdutta67]

This PR implements issue(s) #

Checklist

• I wrote new tests for my new core changes.
• I have successfully ran tests, style checker and build against my new changes locally.
• If this change is deployed to any environment (including Devnet), E2E test coverage exists or is included in this
  PR.
• I have informed the team of any breaking changes if there are any.

[Copilot]

Pull request overview

Adds an optional SHA-256 accelerator spanning the RISC-V guest ABI, ZKC arithmetization, build configuration, and tests.

Changes:

• Implements SHA-256 constraints and custom-opcode dispatch.
• Adds Zig wrappers and selectable accelerated providers.
• Adds vectors, integration tests, and documentation.

Reviewed changes

Copilot reviewed 26 out of 26 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
riscv-guests/README.md	Documents SHA-256 acceleration.
riscv-guests/lineth-accelerators/src/sha2.zig	Adds the SHA-256 opcode wrapper.
riscv-guests/lineth-accelerators/src/root.zig	Exports the SHA-256 API.
riscv-guests/l2-execution/src/zkvm_provide.zig	Selects accelerated or software SHA-256.
riscv-guests/l2-execution/README.md	Documents accelerator options.
riscv-guests/l2-execution/Makefile	Adds SHA2_ACCEL support.
riscv-guests/l2-execution/build.zig	Defines the SHA-256 build option.
prover-ray/zkcdriver/testdata/sha2_run.zkc	Adds a proving test harness.
prover-ray/zkcdriver/testdata/sha2_run.json	Supplies SHA-256 vectors.
prover-ray/zkcdriver/native_modules_test.go	Registers the SHA-256 proving test.
arithmetization/src/test/zkc/sha2/sha2_direct.zkc	Adds direct constraint tests.
arithmetization/src/test/zkc/sha2/sha2_direct.json	Adds direct test vectors.
arithmetization/src/test/zig/src/sha2/sha2_provider.zig	Tests wrapper edge cases.
arithmetization/src/test/zig/build.zig	Propagates accelerator selection.
arithmetization/src/test/README.md	Documents SHA-256 test targets.
arithmetization/src/test/Makefile	Adds SHA-256 build and test targets.
arithmetization/src/main/riscv/utils/constants.zkc	Defines SHA-256 opcode constants.
arithmetization/src/main/riscv/memory.zkc	Adds SHA-256 scratch state.
arithmetization/src/main/riscv/main.zkc	Wires SHA-256 state into execution.
arithmetization/src/main/riscv/interpreter.zkc	Propagates SHA-256 interpreter state.
arithmetization/src/main/riscv/instruction_processing/r_type.zkc	Dispatches the custom opcode.
arithmetization/src/main/lib/sha2/utils.zkc	Implements SHA-256 operations.
arithmetization/src/main/lib/sha2/impl.zkc	Implements hashing and padding.
arithmetization/src/main/lib/sha2/constants.zkc	Defines SHA-256 constants.
arithmetization/src/main/lib/README.md	Marks SHA-256 as implemented.
.claude/settings.json	Adds local command permissions.

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