Bun parity: make TLS config interning race-safe

Corpus position 4141 (fetch-proxy-tls-intern-race.test.ts) exercises the SSLConfig intern/deref ABA/use-after-free race under concurrent proxy fetch+abort churn.

Scope:

• model TLS config registry entries with stable generations and checked refcounts
• ensure a zero-ref destroy cannot delete a newly re-interned generation
• reject double-release/refcount underflow with structured registry errors and composed stacks
• execute the subprocess race fixture deterministically with both driver and probe progress
• add permanent focused coverage and advance the zero-failure frontier.

First failure: fixture result reports driverOk: 0, so the current child path does not exercise a successful driver iteration.

Completed and pushed in ea6dcaa (fix(tls): guard config intern lifetimes).

Implemented:

• TLS config registry entries with monotonic generations and checked refcounts
• re-interning creates a distinct generation after retirement
• zero-ref cleanup deletes only the exact entry/generation being retired
• stale/double releases fail instead of underflowing or touching a replacement
• deterministic race fixture with 256 successful driver iterations, 512 probes, and zero retained entries
• structured TLSConfigRegistryError with stable code, operation, phase, key/generation, cause, and composed stack
• permanent focused regression

Verification:

• upstream TLS intern/deref race regression: 1 passed, 0 failed/unsupported
• focused TLS lifetime regression: pass
• full zig build: pass

Corpus frontier through position 4141: 482 files, 6,876 passed, 281 todo, 172 allowed-empty, 0 failed, 0 unsupported.