Fix simx jalr target alignment

[cassuto]

This fixes JALR target address alignment in sim/simx by clearing bit 0 of the computed target, as required by the RISC-V ISA.
It also adds a regression test under tests/regression/jalr.

[Copilot]

Pull request overview

This PR corrects RISC-V JALR target address alignment in the sim/simx emulator by clearing bit 0 of the computed target address, and adds a focused regression test to prevent the issue from reappearing.

Changes:

• Fix sim/simx JALR next-PC computation to enforce (rs1 + imm) & ~1.
• Add a new tests/regression/jalr test (host + kernel + startup) that fails deterministically when JALR doesn’t clear bit 0.
• Wire the new regression into the top-level tests/regression/Makefile targets (build/run/clean).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sim/simx/execute.cpp	Masks off bit 0 when computing JALR target PC to match the ISA requirement.
tests/regression/Makefile	Adds the new jalr regression to aggregate build/run/clean targets.
tests/regression/jalr/Makefile	Build/run glue for the new regression test.
tests/regression/jalr/start.S	Minimal startup that calls main, dumps perf, and terminates.
tests/regression/jalr/main.cpp	Host-side test runner that launches the kernel and validates per-core pass signatures.
tests/regression/jalr/kernel.cpp	Kernel that uses an intentionally odd JALR target to distinguish correct vs incorrect alignment behavior.
tests/regression/jalr/common.h	Kernel argument struct for passing the destination buffer address.

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