Port Bun shell pipeline stack corpus

Tracks the focused Bun parity slice under #66.

Scope:

• activate upstream js/bun/shell/pipeline_stack.test.ts
• execute each TestBuilder pipeline in an isolated linked native Bun shell child
• cover builtin exit propagation, subprocess pipelines, deep chains, nested subshells, conditionals, variable assignments, EPIPE cleanup, and command substitution
• give TestBuilder collision-free physical temp directories
• normalize the linked debug runtime’s _stream_wrap warning without suppressing command stderr
• add a dedicated native regression and verify the full shell directory

Implemented and pushed in 6abc05b2e5d44713ae914eca66dabe66385ad15c (feat(test): run bun pipeline stack corpus).

Verification:

• exact pipeline_stack.test.ts: 63 passed, 0 failed, 0 unsupported, 0 todo
• full js/bun/shell directory: 411 passed, 0 failed, 0 unsupported, 3 todo
• repository build and git diff --check: passed

The global home_test rerun remains tracked on #81 because other workspace compiler/test jobs are still consuming enough memory to make the JSC suite process-kill rather than produce a deterministic test result.

Completed on main in 6abc05b2e5d44713ae914eca66dabe66385ad15c.

Follow-up regression from active Home executable discovery is fixed in #89 / d37a98a. The harness no longer duplicates absolute home-debug paths; pipeline_stack.test.ts again passes 63/63, and the complete home_test gate passes with zero failures.