macOS: 3 tests in test/gstack-design-detect.test.ts fail deterministically — $TMPDIR vs its realpath (/var -> /private/var) #2841

Summary

Three tests in test/gstack-design-detect.test.ts fail deterministically on macOS because the test
builds its expected path from $TMPDIR while the code under test has already resolved that path to
its real location. On macOS $TMPDIR is /var/folders/… and /var is a symlink to /private/var,
so the two strings differ by that one prefix even though they name the same directory.

This is not flaky — it fails on gstack's own serial flaky-retry as well.

Environment

• gstack 1.84.1.0 (71f6048e)
• macOS 26.6.2, arm64
• bun 1.3.14

Failing tests

From bun run test:free (shard 5/7 failed with 3; the GSTACK_FREE_RETRY_FLAKY=1 serial retry
failed with 4):

• probe > ignored rules and files are the union of config.json and config.local.json; malformed config is reported
• scan: option-like bases and page-controlled inline ignores > from HOME (no repository) HOME-rooted installs are READY, HOME files are refused as targets, and a dump still scans without inline ignores
• scan: option-like bases and page-controlled inline ignores > the whole-scan budget stops a huge target set instead of grinding batch after batch (retry only)
• install: the one download gstack makes, after consent > downloads from a mirror, verifies the checksum, installs under IMPECCABLE_HOME, receipts the fetch first, and the probe finds it

Reproduction

$ echo $TMPDIR
/var/folders/qp/…/T/
$ cd "$TMPDIR" && pwd -P
/private/var/folders/qp/…/T

Any assertion that joins $TMPDIR into an expected path and compares it against output the
implementation has realpath'd will differ by the /private prefix.

Confirmation

Running that test file with a TMPDIR that is already its own realpath:

TMPDIR="$(cd "$TMPDIR" && pwd -P)" bun test test/gstack-design-detect.test.ts

No other test file changes result. I specifically A/B'd the path-security tests, since a naive
"resolve all temp paths" change could plausibly weaken them — they are unaffected:

(One caveat worth knowing if you reproduce this: browse/test/path-validation.test.ts and
temp-dirs.test.ts resolve paths relative to cwd and treat the temp dir as allowlisted, so running
them from a checkout that itself lives under /tmp produces three unrelated failures. Clone under
$HOME to reproduce cleanly — that cost me an hour of chasing a phantom.)

Suggested fix

Resolve $TMPDIR to its real path where the tests construct expected paths (or have the tests
compare realpath-to-realpath rather than raw strings). Happy to send a PR if you'd like it in a
particular shape — I didn't want to guess at your preference between fixing it in the individual
assertions vs. a shared test helper.

Why this matters beyond one machine

CI presumably runs on Linux, where /tmp has no such symlink, so this is invisible there and hits
every macOS user. On my side it was surfaced by an auto-updater that runs the free suite as a gate
before accepting a new gstack — with this bug, every release rolls back on macOS.