Chunk 3 of 4 staged (Batch E). Commit message:

tests: add SmartNullTest and DebugTest (new suite batch E)

- SmartNullTest: how SmartNulls arise, chaining across property/array/
  method access, __call delegation to the mode-matching array class and
  to SmartString for string-only methods, metadata passthrough, loose
  == '' pin, json, zero-iteration foreach, write guards
- DebugTest: debug() levels with full-output pins on a known fixture,
  print_r/var_dump showing just the array data, SmartNull's [value]
  dump, help() anchors, xmp wrapping under CLI

Questionable behaviors pinned with // REVIEW comments: property writes
on SmartNull land as real dynamic properties (no __set), ->set() is
silently discarded while ['key'] = throws, __call drops mysqli
metadata, unknown-method errors name the delegate class.

Author: daveedis