Preserve upstream skip, TODO, and shell workload accounting

Parent tracking: #202 and #66.

Four reduced-runner rewrites currently diverge from the pinned Bun corpus:

• shell which shrinks the upstream 100,000-character command lookup workload to 2,048 characters;
• memfd-disabled converts two Linux-only test.skipIf registrations into macOS passes;
• different-directory snapshot converts an upstream test.todo into a pass;
• RuntimeError uses an exact fixture import substitution even though the shared static import lowerer can provide the module.

Acceptance criteria:

• Remove all four fixture-specific rewrites and their dispatch behavior.
• Preserve the unchanged 100,000-character shell workload.
• Preserve two platform skips and one upstream TODO on macOS rather than promoting them.
• Lower RuntimeError through the shared supported-module path.
• Match pinned Bun counts in Debug and ReleaseSafe, strict and routed modes.
• Keep fixtures byte-identical and do not alter assertions, deadlines, or workloads.

Completed in fd95751.

Removed the shell which workload reduction, memfd platform-skip promotion, snapshot TODO promotion, RuntimeError fixture rewrite/dispatch, and the memfd-specific synthetic child-result hook. RuntimeError now lowers through the shared supported-module import path.

Final rebased validation:

• pinned Bun: 3 pass, 2 skip, 1 todo, 0 fail, 4 expect calls
• Home Debug strict: 3 pass, 2 skip, 1 todo, 0 fail, 4 expect calls
• Home Debug routed: 3 pass, 2 skip, 1 todo, 0 fail, 4 expect calls
• Home ReleaseSafe strict: 3 pass, 2 skip, 1 todo, 0 fail, 4 expect calls
• Home ReleaseSafe routed: 3 pass, 2 skip, 1 todo, 0 fail, 4 expect calls
• Debug and ReleaseSafe builds succeeded
• all four upstream files are byte-identical to the pinned Bun checkout
• the original 100,000-character shell workload remains intact
• no assertion, deadline, fixture, or workload weakening; no heap snapshots