PTR-008: mark/+js/+beagle/ argv offset-math, Fork/Join

The 2026-06-15 audit found beagle/ does execvp argv assembly with hand-rolled offset↔pointer math (duplicated 3×) and hand Fork/Join; mark/ hands back a bare interior pointer from a marker scan; js/ builds slices from typed-array base+bytelength. Behavior-preserving, one worktree; js parts may be CI-unbuildable (JSC), note like the MEM js tickets.

Input

Context — beagle/

Context — mark/ & js/

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WIP

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