AST builder: improve diagnostics and logging for silent parse fallbacks

[YasiruRF]

Problem

Several visitor methods return None or fallback values silently, making parser/debugging issues hard to trace.

Proposed change

• Add structured debug/warn logging for fallback paths in AST builder.
• Include source location in diagnostics.
• Keep normal successful path noise-free unless verbose/debug enabled.

Acceptance criteria

• Fallback conditions produce actionable logs in verbose mode.
• Logs include file/line/column where available.
• Existing behavior remains backward-compatible when logging is disabled.