Add fork choice write lock tracing context

[peter941221]

Issue Addressed

Part of #8147.

Proposed Changes

Adds source-specific tracing spans around production fork-choice write-lock/update paths so lock wait spans have useful parent context in traces.

This is intentionally behavior-neutral:

• does not change fork choice locking semantics
• does not move DB writes outside the lock
• does not introduce a custom lock wrapper
• does not alter fork choice, payload, or block import logic

Covered paths include block import, payload envelope import, attestation/slashing application, recompute-head, finalization prune, and execution payload validity updates.

The instrumentation uses a frequency-aware span level split:

• low-frequency or issue-critical paths use info_span! so parent/source context is visible alongside the existing default #[instrument] fork-choice write-lock span in info-level tracing configurations
• high-volume attestation application paths use debug_span! to avoid adding info-level trace noise on gossip-heavy paths

Additional Info

This follows the instrumentation-first direction discussed in #8147.

Verifications Performed

• cargo fmt --check
• cargo check -p beacon_chain
• git diff --check