test: explain the retry of the VarsThrew location test, and track its removal

[kynnyhsap]

What

• The comment on { retry: 2 } in packages/envi/src/core/Envi.test.ts now names:
  • the platform (Bun 1.4.2 on Linux x86_64);
  • the likely cause, Async-thrown Error loses its message from error.stack when GC runs before first .stack access oven-sh/bun#34398;
  • the upstream fix, Preserve a pending exception across the GC stack-trace finalizer oven-sh/bun#33584, which is not in a Bun release yet;
  • issue Remove the retry of the VarsThrew location test after a Bun release with oven-sh/bun#33584 #28, which tracks the removal of the retry.
• Issue Remove the retry of the VarsThrew location test after a Bun release with oven-sh/bun#33584 #28 holds the evidence and the steps to remove the retry.

Investigation

• Only 2 real failures are known. Both were on the x86_64 Linux images in CI, right after the upgrade to Effect rc.118.
• 0 failures in more than 600 local runs without the retry:
  • macOS arm64 and the arm64 linux-bare image;
  • with and without CPU load;
  • on Bun 1.3.0 and 1.4.2;
  • with forced garbage collections.
• Neither Effect rc.118 nor Envi's stack parsing explains an empty stack.
• The retry stays until a Bun release includes the fix.

A subagent also reported signal e2e failures in tests/e2e/run.test.ts under heavy CPU load. They did not reproduce: 4 runs of the file and 3 runs of the full e2e suite under 40 busy processes all passed.

bun run verify passes.