feat(setup): S2-engine — companion manifest engine (one-click-installer)

[artyhoo]

Summary

S2 sub-wave 2/3 of the one-click-installer umbrella (plan Task 3, after S1 #442). Adds the sourceable function lib setup.d/engine.sh (companion_step <name> <detect> <install> <kind> <mode>, detect-first, official-installer-only, no pin) + its test. Engine takes args — manifest parsing stays in Task 5/S3, so the sibling tab-delimiter change does not touch this lib.

Changes

• New setup.d/engine.sh (47 LOC) — verbatim from the plan (byte-identical, diff against plan block empty); ENGINE_LIB_ONLY=1 guard mirrors install.sh:50-57 INSTALL_SH_LIB_ONLY precedent.
• New tests/install-sh/engine.test.sh — 5 assertions, TDD red (PASS=3 FAIL=2, source error) → green PASS=5 FAIL=0. One-flag deviation from plan, mutation-checked: dry-run case uses grep -qx (whole-line) instead of grep -q — the plan's own engine prints [dry-run] would install fake: echo SHOULD_NOT_RUN (command embedded mid-line by design per spec «print the plan»), so the plan's substring grep false-positives against the plan's own engine (verbatim pair yields PASS=4 FAIL=1). Detector non-vacuity proven: a mutant that evals install in dry-run emits a bare SHOULD_NOT_RUN line → caught.

Prior-art consult

• Not a capability commit (outside packages/, no new deps; pre-push gate scoped to packages/ per prior-art.ts:111-112). Lineage trailer carried anyway: Prior-art: aif-handoff installMcpServer precedent (spec §11; same pattern as commit ecb6457 / PR #311).
• No new capability area; no SSOT row edits (S4 Task 6 owns row docs(skills): sync shipped prose to C-1 resolution (close DN-1) #84 update).

Test plan

• bash tests/install-sh/engine.test.sh → PASS=5 FAIL=0 (worker verify-trace; red phase + mutation falsifier pasted)
• bash -n setup.d/engine.sh → exit 0
• Sibling regression: no-companion-blocks.test.sh → PASS=4 FAIL=0
• CI ci-success (runs on PR)

§1.7 Self-discipline check (REQUIRED if PR touches discipline-bearing files)

§1.7 Skipped: sourceable bash lib + its test only; no rules, principles, templates or discipline-bearing docs touched.

🟢 Простыми словами

Добавлена функция-«движок»: ей говорят «компаньон такой-то, проверь так, поставь так» — она сначала проверяет, не стоит ли уже, спрашивает разрешения (или берёт --yes), и только потом ставит официальной командой. В режиме --dry-run ничего не ставит, только показывает план. Покрыто 5 тестами.

Observations (out of scope)

• Plan Task 3 internal bug (test substring-grep vs engine dry-run output) — fixed minimally on the test side here; if maintainer prefers cutting the command from dry-run output instead, it's a one-printf change but impoverishes Task 9 e2e expectations. Flagged for the S2→S3 stage-gate.
• Engine interactive branch (read -r) has no plan-provided test — untested by design here; S3/S5 e2e covers the flow.