refactor: use immutable history cache

[mikehostetler]

Closes #252

Summary

• replace the History ETS table, owner, and heir lifecycle with one immutable :persistent_term entry
• publish cache replacements atomically behind the existing load lock while keeping reads process-free
• add deterministic concurrent-load, reload, clear, missing, and corrupt bundle coverage
• document the bounded cache identity and :persistent_term update tradeoff

Compatibility

• preserves the public LLMDB.History APIs, return values, and history bundle format
• requires no supervised cache process

Validation

• mix test (858 tests, 41 doctests)
• mix quality
• focused history and tooling boundary tests