fix(dfns): fix multiple issues

[wpbonelli]

Fix spec/schema/migration issues surfaced in #339. Regenerate v2.0.0.dev2/dev3 snapshots and schema.json. Update dfnspec.md/dfns.md and add tests.

Block header fields

Introduce a header field corresponding to v1's block_variable, e.g. BEGIN PERIOD <iper>. Previously v2 dropped these. Make Block.repeats a derived property (header is not None) rather than a stored field.

File record fields

Restore composite file record fields. Collapsing these into a single File field unnecessarily complicated things: migration assumed every file record had shape record <keyword> <filein|fileout> <path>, but two other shapes exist in the DFNs:

• no leading keyword with a trailing flag after the path (e.g. utl-obs's output)
• multiple leading keywords (e.g. gwf-sfr's cross_sectionrecord).

These are more naturally represented as composites containing zero or more keywords plus a File field. Rename File.mode to direction accepting "in"/"out" — "filein"/"fileout" are MF6 format (i.e. rendering) details.

Rendering

• Fix tagged handling when creating Array fields during migration, and when rendering string/integer/double/array subfields of a record (previously only File respected tagged on render).
• Render header on the block's BEGIN <NAME> line.