fix(update): make parallel updater deterministic

[ss-o]

Description

Make Zi's parallel updater deterministic and failure-aware:

• retain every child PID until it is reaped;
• enforce the configured concurrency limit exactly;
• flush snippet and plugin phases independently;
• emit captured output in launch order;
• propagate child and output failures; and
• reap children and remove temporary output after normal completion or handled interruption.

The focused regression suite covers the original trailing-job failure, concurrency, phase isolation, deterministic output, failure propagation, and signal cleanup.

Related issues

Closes #114

Type of change

• fix - bug fix (non-breaking)
• feat - new feature (non-breaking)
• feat! / fix! - breaking change
• perf - performance improvement
• refactor - code change with no functional impact
• docs - documentation only
• test - test addition or correction
• build - build system or dependency change
• ci - CI/workflow change
• style - formatting with no behavior change
• chore - maintenance / dependency bump
• revert - revert of an earlier change

Checklist

• Ordinary work targets next; only same-repository hotfixes target main
• This is not a next to main promotion
• Commit messages follow Conventional Commits format
• No Co-authored-by trailer credits a bot, AI agent, or automation
• I have read the contribution guidelines
• Existing tests pass
• No documentation change is required for this internal bug fix

Verification

• zsh -f tests/parallel-update.zsh: 6 scenarios passed.
• Existing Zi integration suites: 61 scenarios passed.
• Native syntax and isolated zcompile -U: all 29 Zsh files passed. The pre-existing lib/zsh/rpm2cpio.zsh:54 warning remains.
• actionlint .github/workflows/zsh-n.yml: passed.
• git diff --check: passed.
• Local commit hook: passed.

Migration plan

Not required. No destructive public-contract impact is expected.

Agent handoff

• Status: Ready for review
• Current state: The implementation and focused CI coverage are complete on bug-114-parallel-update.
• Blockers: None.
• Next steps: Review and run required repository checks.
• Related tracker/issues: fix(update): wait for parallel jobs and propagate failures #114