SpawnWithTimeout derives the process-group isolation decision twice

[jcouball]

Context

Refactoring follow-up from a review of the v4.0.5 release changes (#163). (Finding F3 from that review.)

Problem

In Commands::SpawnWithTimeout, the decision "did this class isolate the subprocess into its own process group?" is derived twice:

• #process_group_options computes it directly (timeout can fire, and the caller set neither pgroup nor new_pgroup)
• #isolated_in_new_process_group? re-derives it from the merged spawn options via #process_group_leader? plus repeated :not_set checks

The two are equivalent in every combination of timeout and caller-supplied pgroup options, but a reader has to prove that. #process_group_leader? also recomputes spawn_options (rebuilding the merged hash) each time it runs on the kill path.

Proposed refactoring

• Define isolated_in_new_process_group? as !process_group_options.empty?, making #process_group_options the single source of truth for the isolation decision
• Capture the effective spawn options once at spawn time (memoize or store in #call) so the kill path inspects the options actually used rather than recomputing the merge

#process_group_leader? must keep honoring caller-supplied pgroup: true / pgroup: 0 / new_pgroup: true for the group-kill decision — only the isolation check collapses.

Acceptance criteria

• Behavior-preserving: no public API or documented behavior changes
• One place computes whether this class added the process-group option
• Timeout group-kill, caller-pgroup passthrough, and abandoned-wait cleanup tests pass unchanged

Blocked by

None — can start immediately.

[jcouball]

Implemented by PR #200 (merged as 5d8ac7d). One deliberate refinement to the proposal: isolated_in_new_process_group? is !process_group_options.empty? && process_group_leader? rather than the bare emptiness check — the leader check (a Copilot review finding) keeps the abandoned-wait cleanup from killing a child when a subclass spawn_options override removes the added group option. Both sides of that corner are characterized by tests.