fix(benchmark): harden governed density replay teardown

[tcconnally]

Summary

Follow-up hardening for the governed live Vault density replay restored by #1001 for #999.

• Makes startup identity failure abort the entire freshly-owned process group, reaping the leader and adopted descendants before releasing state.
• Routes the corpus-only benchmark path through the shared byte-bounded corpus loader.
• Adds regressions for startup identity cleanup and corpus byte limits.

Verification

• Independent exact-tree security/logic review passed with no findings.
• Reviewed patch SHA-256: 730914a409eedd00e4b787cdd0a3106eb29f5de72de66d39005532658bb3d5e5.
• Touched-module suite: 23 passed, 3 skipped.
• Live default density and corpus-only density: network_calls=0, all production metrics 1.0000.
• Live six-probe replay: hit@5=1.0000, MRR=0.9167, task 0.8333, decoder 0.1667.
• Ruff, compilation, YAML parsing, README count, and diff checks passed.

This is a follow-up to the already-merged implementation in #1001; it does not change the resolved state of #999.