Makefile: parameterize hardcoded /tmp test paths ($(TESTTMP)) — concurrent gates corrupt each other

• Type: chore/infra (Makefile test recipes — shared ground). Track A.
• Found: 2026-07-08 by Track T: a local --tier full gate and the borg watcher's gate ran concurrently; both self-host chains write the same literal /tmp/pascal26-self/next/fixedpoint, and the local run failed byte-identity with a clean tree (interleaved binaries).

State

Makefile test recipes hardcode ~1700 distinct /tmp/<name> outputs (3037 occurrences). Any two concurrent test runs on one box race on all of them.

testmgr already contains the runtime fix: it rewrites literal /tmp/ to a private per-run scratch dir when executing job scripts, so concurrent testmgr runs (dev gate + watcher, or two watchers) are isolated by construction. NOT covered: plain make test / make test-smoke run by a human or agent while a watcher gate is active on the same box.

Wanted

Mechanical parameterization: TESTTMP ?= /tmp at the top, recipes use $(TESTTMP)/.... Behavior-identical by default; then make test TESTTMP=$(mktemp -d) (or exporting it in CI docs) makes manual runs safe too, and testmgr can drop its string rewrite. Sed-able but sweeping — needs a careful pass + full gate + self-host, and should land in a quiet window (every track's recipes are touched).

Gate

make test + self-host byte-identity green with TESTTMP unset AND with TESTTMP set to a scratch dir; testmgr full tier green.