feat: Introduce structured logging system (GStreamer GST_DEBUG style)

[justinjoy]

Implement flexible, section-based logging system similar to GStreamer's GST_DEBUG.

Features:

• Log levels: TRACE(5), DEBUG(4), INFO(3), WARN(2), ERROR(1)
• Section filtering by category: JOIN, CONSOLIDATION, ARRANGEMENT, EVAL, SESSION, etc.
• Environment variable syntax: WL_LOG=JOIN:4,CONSOLIDATION:3
• Runtime enable/disable without recompilation

Example usage:

WL_LOG=JOIN:5 ./bench_flowlog  # Trace all JOIN operations
WL_LOG=CONSOLIDATION:3 ./bench  # Show consolidation warnings+
WL_LOG=*:2 ./bench             # All errors and warnings

Benefits:

• Development/debugging: Fine-grained control
• Production: Zero overhead when disabled
• Performance measurement: Clean output control
• Replaces ad-hoc fprintf scattered throughout codebase

This is a future enhancement. Current approach: Simple getenv() gating (#277).