logging: reduce default boot logspam; audit log levels and remove bring-up remnants

[emilf]

Theseus boot output is currently too noisy (logspam). This wastes precious context when using AI tooling (Cursor/OpenClaw) and also makes it harder for humans to spot true warnings/errors.

Goals:

• Reduce default boot log volume significantly.
• Audit logs so messages are at the correct level (TRACE/DEBUG/INFO/WARN/ERROR) and not remnants from bring-up.
• Keep high-signal early-boot diagnostics (esp. MSI-related critical errors) prominent.

Scope ideas:

• Introduce/standardize feature flags and rate-limiting for repetitive logs (e.g., once-per-boot / once-per-device / exponential backoff).
• Ensure fast-path interrupt handlers do not emit noisy logs by default.
• Default config should be quiet; provide a single switch to re-enable verbose tracing when debugging.

Workflow note:

• Until tmux interface is working, logs can be captured via ./startQemu.sh headless 10 and/or piping output to a file for review.

Acceptance:

• A cleaned-up boot log at default settings (measurably fewer lines) while retaining critical warnings/errors.
• Document the logging policy and the knobs to increase verbosity.