Implement config.splitting, including the 1.2.0 overlap field

[jzonthemtn]

config.splitting is not read anywhere, so a policy that asks for splitting is filtered as one piece: enabled, threshold, method, and the 1.2.0 overlap all have no effect.

Acceptance Criteria

• A document over threshold is split by method.
• Each chunk includes the trailing overlap characters of the previous chunk, defaulting to 0.
• Spans duplicated at a seam are de-duplicated.

[jzonthemtn]

Closes a gap against the PhiSQL specification. RFC: philterd/phisql#19.