Port Bun shell subprocess hang regressions

Tracks the focused Bun parity slice under #66.

Scope:

• activate upstream js/bun/shell/shell-hang.test.ts
• execute all six upstream fixtures through Home’s native spawn bridge
• preserve the upstream success/failure exit-code expectations without delegating the corpus result to bun test
• provide the repository Pantry toolchain in the child environment so which node matches the test harness contract
• add a dedicated native regression and verify the complete shell corpus

Implemented and pushed in 2b704dfbad28afcd85e5de2a3d1b459266e7ce4e (feat(test): port bun shell hang regressions).

Verification:

• exact shell-hang.test.ts: 6 passed, 0 failed, 0 unsupported, 0 todo
• each of the six fixture processes was also exercised directly under a 3-second hard timeout, producing the expected three nonzero and three zero exits with no hangs
• full js/bun/shell directory: 348 passed, 0 failed, 0 unsupported, 4 todo
• repository build and git diff --check: passed

The machine-wide home_test rerun remains tracked on #81 while concurrent workspace compiler processes are active.

Completed on main in 2b704dfbad28afcd85e5de2a3d1b459266e7ce4e.