Delivers the property coverage promised in #1511's test scope but not landed
(follow-up #1676, epic #1507 / ADR-1508). Adds
tests/enh-1676-path-prefix-collapse-idempotency.property.test.cjs covering:
(A) $HOME-collapse invariant for _computePathPrefix — global-under-home
projects to $HOME/<suffix>/ (exact equality, not substring, so short
homes like /root or /a do not false-positive); opencode is the
documented exception (absolute form, never $HOME).
(B) backslash->posix invariance (#1615 Windows path-leak fix).
(C) path-rewrite idempotency for _applyRuntimeRewrites across the
path-rewriting runtimes (f(f(c)) === f(c); attribution held at
undefined to isolate the path axis). Non-vacuous: a sanity assertion
proves the first pass actually rewrites the seed ~/.claude/ refs.
Pure test addition — no production code changed. Uses the shared
fast-check-setup (seed=42, numRuns=200). Closes#1676
