v1.6.0

Turn the spellings an exporter uses for a missing value into real nulls.

New

• CsvConfig(nullValues: {'NULL', 'NA', 'N/A'}) decodes those fields as null
  instead of as their own text. Exports frequently write a missing value that
  way rather than leaving the field empty, and it otherwise arrives as a string
  that every downstream check has to special-case.

The rule is narrow on purpose, because it is what keeps the decoders in step:

• Matching is exact and case-sensitive, so list every spelling the file uses.
• A quoted field is never matched, so a genuine "NULL" in the data survives.
• Only a value that would otherwise read as text is eligible. An entry that
  type inference turns into a number or a bool (0, false) never reaches the
  check, so putting one in the set has no effect.
• decodeStrings is unaffected, since its return type cannot hold a null.

This was deferred twice over the one-parsing-semantics invariant. The suite now
checks batch and streaming agree on every case, with the streaming input split
at every possible offset, so a value straddling a chunk boundary is exercised
rather than assumed.