perf(ci): collapse the per-file and per-test Zsh jobs and gate next on them #598

Problem

zsh-n.yml is about 82% of this repository's runner time and shapes every pull request's time to green.

• Each run starts about 126 jobs: one zsh -n plus zcompile job per source file (about 86) and one job per focused test (about 37).
• Every job installs Zsh with apt (10 to 22 s) to run a check that takes under a second. On PR fix(light): load classic zi light without tracking #594 (run 37629187706) the 128 jobs waited a median 86 s and up to 167 s for a runner, which also delays the required checks queued behind them.
• promotion-readiness.yml already runs the same syntax and compile pass over every source in one job.

Separately, the next ruleset requires only Validate Commits, Validate PR Title, Validate Branch Name, check, Public contract impact and Analyze (actions). No functional test gates a merge into next: the Zsh jobs and ZD Integration are not required, and both are path-filtered, so they could not be required as they stand. A red test job does not block the merge button.

tests/ci-registration.zsh exists because a test that is not wired into CI never runs and nobody notices (#549). Any consolidation has to keep that guarantee.

Change

• Run the syntax and compile pass over all sources in one job, as promotion-readiness.yml does.
• Run the focused tests in a small number of jobs, each installing Zsh once and running its tests in sequence with a per-test result, instead of one job per test.
• Keep tests/ci-registration.zsh meaningful by checking the new registration shape.
• Add an always-running aggregate job (no workflow-level path filter) that succeeds only when the syntax and test jobs succeeded or were legitimately skipped, so it can become a required check for next. Changing the ruleset itself is a separate maintainer action.

Measurements come from the Actions API for runs between 2026-10-01 and 2026-10-07.