Add Fuzz Tests for RVM Instructions

[yuqiliu617]

Add fuzz tests for the M instruction set (multiplication and division)

[Copilot]

Pull request overview

Adds a new fuzz target covering the RISC-V M extension (mul/div/rem) and refactors existing fuzz targets to share common helper utilities.

Changes:

• Added isa_m libFuzzer target to differential-test M instructions across Spike, the interpreter core, and the asm core.
• Extracted shared fuzz utilities (byte deque + register equivalence assertion) into fuzz_targets/utils.rs and reused them in isa_a/isa_b.
• Updated fuzz harness configuration and ignore patterns (isa_m bin entry; ignore coverage/ output).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
fuzz/fuzz_targets/utils.rs	New shared helpers for deterministic byte consumption and register comparison across engines
fuzz/fuzz_targets/isa_m.rs	New fuzz target for M-extension instructions (MUL/DIV/REM + W variants)
fuzz/fuzz_targets/isa_b.rs	Refactor to reuse shared Deque and common register comparison helper
fuzz/fuzz_targets/isa_a.rs	Refactor to reuse shared Deque and common register comparison helper
fuzz/Cargo.toml	Registers new isa_m fuzz binary target
fuzz/.gitignore	Ignores generated coverage/ directory

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