fix: reject template options where chunk_overlap >= chunk_size

[Frankie-Xu]

Problem

YAML options.chunk_size and options.chunk_overlap have no cross-field check. When overlap is greater than or equal to size, character splitting degenerates (empty or non-progressing chunks).

Fix

Validation runs on the unified Options model used by parse_option:

• chunk_size, when set, must be >= 1
• chunk_overlap, when set, must be >= 0
• when both are set, chunk_overlap must be strictly smaller than chunk_size (equality fails)

Constructing an AutoType by hand is out of scope.

Tests

Parser tests cover valid pairs, overlap equal to size, overlap greater than size, non-positive size, negative overlap, and omitted fields. Bundled presets still load and parse_option still succeeds. HE-T rules are unchanged.

Compatibility

Templates that omit both fields behave as before. Rebased onto v0.9.0 / d8fce60. Does not touch ingest or provenance.

Made with Cursor

[yifanfeng97]

Merged — verified locally that degenerate combos (overlap == size and overlap > size) are rejected with a clear author-facing message while valid pairs are unaffected. No bundled preset violates the constraint. One follow-up idea: he template validate doesn't currently exercise parse_option, so these invalid combos still pass validate and only fail at Template.create — surfacing the check there too would catch them even earlier.