[Feature]: Harden extension export audio path with safety guardrails

[crbender]

• I have searched the existing issues

Is your feature request related to a problem?

Yes. Extension-provided asset paths and high-volume event streams need safeguards in export context.

Describe the solution you'd like

Add security and resilience guardrails for extension export audio:

1. Canonical path validation (extension-root constrained)
2. Volume clamping
3. Event count cap / sanity limits
4. Missing or unreadable asset handling with warning and skip

Describe alternatives you've considered

Failing hard on first invalid event. Rejected, too disruptive for long exports and mixed-quality extension ecosystems.

Additional context

Goal is safe defaults without hiding problems.

Acceptance criteria

1. Path traversal attempts are rejected.
2. Invalid/missing assets do not fail full export.
3. Warnings are surfaced with enough detail to debug.
4. Tests cover guardrails and failure modes.

[crbender]

Tracking under parent issue #572 for sequencing and rollout context.