P38 cleans up the arch-test plumbing

Reading through the existing harness while writing the arch-test page made it obvious that the previous framing was off. The repo has been running the upstream riscv-arch-test framework with a custom DUT plugin since P15. The “ACT4” name is just upstream’s act generator; the plugin is the rvmodel_macros.h+link.ld pair that has been quietly living under projects/26_rv32i_act4_probe/act4/.

P38 fixes this:

• Moves the plugin into a named directory: projects/38_arch_test_official/arch_test/.
• Adds one aggregate make arch_test target that runs all four scoped batches and writes a single SWEEP.md.
• Leaves the RTL alone - same top.sv as P37, only mimpid bumps.
• Adds a signature-dump testbench (tb_arch_test.sv) as future infrastructure for the canonical signature-flow path. Not wired into the default batch.

Honest framing: this is not credibility we did not already have. P37 already passed all four scoped batches. P38 is a packaging rung - same PASSes, presented through one entry point against one named plugin.

The benefit comes later. P39 (Zicntr per the roadmap) inherits the plugin without copying it. The FreeRTOS port rungs after that don’t need to think about it at all.