Migrate to bun runtime

[tomusdrw]

[tomusdrw]

Not planned, see details in #951 (comment)

Excerpt:

• Bun runs the fuzzer 1.2× to 2.4× slower than node depending on workload.
• The gap is dominated by JSC's WebAssembly engine being slower than V8's, not by JS execution. Pure-TS workloads show only a ~1.2–1.4× gap; WASM-heavy workloads (default ananas PVM) show ~2×.
• The migration also produced material operational cost (workarounds, version bumps, sharding, JIT-off for tests, native-package gymnastics).