Fix persona response storm backpressure

[joelteply]

Summary

• enforce Runtime ModuleConfig.max_concurrency with per-module semaphores
• serialize cognition, ai_provider, and embedding modules to prevent native threadpool fanout during persona bursts
• debounce normal same-room PersonaInbox chat wakeups so Rust channel consolidation gets a conversation chunk instead of one wakeup per message
• hard-gate undirected non-human echo storms while preserving human messages and direct mentions

Validation

• npm stop cleared the runaway local Continuum process before patch validation
• npx vitest run system/user/server/tests/validation/PersonaInboxDebounce.test.ts --reporter=verbose
• ./node_modules/.bin/tsc --noEmit --project . --pretty false
• cargo check --manifest-path src/workers/Cargo.toml -p continuum-core --features metal,accelerate from a clean worktree with vendor/llama.cpp initialized

Notes

• Pre-existing Rust warnings remain; no new Rust hard errors.
• The first temp worktree lacked initialized vendor/llama.cpp, so initial cargo/pre-push attempts failed on missing CMakeLists.txt. Clean worktree validation passed after submodule init.