test(process): pin argv join-collision evidence for #724

[rwrife]

Summary

• extends process_spawn_argv_roundtrip with an explicit collision marker showing that two distinct argv vectors can collapse to the same v0 space-joined raw_args payload
• hardens build/scripts/test_process_spawn_argv_roundtrip.sh to assert required sub-markers and fail on missing marker drift
• records an execution plan in plans/2026-08-05-process-spawn-argv-wire-evaluation.md for post-M7-TOOLCHAIN-007: unsigned-run wiring + m7_toolchain acceptance suite (in-OS toolchain, refs #403) #410 runtime evaluation of whether length-prefixed argv framing is required
• updates plan index + test target comments to keep roadmap/test intent aligned

Why this slice

Issue #724 is intentionally gated on runtime signal from #410. This PR lands starter instrumentation/evidence runway without changing ABI/runtime behavior yet.

Test Plan

• ./build/scripts/test.sh process_spawn_argv_roundtrip
• bash ./build/scripts/validate_plans_index.sh

Next steps (tracked in #724)

• collect real cc invocation corpus once M7-TOOLCHAIN-007: unsigned-run wiring + m7_toolchain acceptance suite (in-OS toolchain, refs #403) #410 acceptance harnesses run end-to-end
• decide whether an additive length-prefixed argv transport is required based on observed ambiguity

Refs #724