🐛 Arm process kill-teardown before spawning the child

[taras]

Fixes #236

Problem

A scope halted while exec is acquiring its child process can leave the child — and its whole process tree — running with no owner. The OS process was created first and the ensure that terminates it registered last, with several suspension points in between. A halt landing on any of them discards the remaining instructions, so the kill-teardown never arms and the orphan runs to completion. The window cannot be guarded from the caller's side: it is inside the single yield* exec(...), so the caller never receives a Process to clean up with.

Fix

Both createPosixProcess and createWin32Process now:

• register a guard teardown before the process exists, and create the process in the same synchronous continuation — a halt lands either before the spawn (nothing to clean up) or after the teardown is armed, never between;
• resolve exit/close/error through Node listeners attached in the spawn's continuation instead of spawned effection watchers, so the guard can always join on process exit no matter where a halt lands;
• keep the primary teardown at the end of acquisition unchanged in position and semantics (it still runs first at teardown, while the stdio pumps are alive, preserving the drain-before-halt ordering from process: no way to observe exit — join() settles on close (exit + stdio EOF), which can outlive the command #228). The guard stands down once the primary teardown has begun, and joins on process exit alone since the pumps may never have been wired.

The shutdown-mode logic (graceful / forced / policy function) is factored into a single shutdownProcess(join) used by both teardowns, so the orphan-window path honors the configured shutdown semantics too.

Regression test

Effection's reducer interleaves same-generation routines one instruction per turn. The new test spawns a halter one scope generation deeper than the exec task — the same generation as the acquisition's internal routines — and sweeps the number of turns before task.halt(), deterministically landing the halt on every suspension point of the acquisition sequence, then asserts via ps that no child survives. Against the previous code this reliably orphans the child (halts landing after the spawn but before the teardown registration); with the fix, all sweep positions stay clean. No timers, no load dependence.

Stacked on #230 (agent/process-exited).

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npm i https://pkg.pr.new/@effectionx/process@237

commit: 132947e