Add a `length` / word-count scorer with min/max bounds

[abhishekmanyam]

Summary

Add a length scorer that passes when the output's length falls within min/max bounds (characters or words). Small, self-contained, genuinely useful for "don't ramble" / "must be non-empty" guards.

YAML sketch

scorers:
  - type: length
    unit: words      # or: chars
    min: 5
    max: 200

Acceptance

• One file in crates/evalcore-scorers/, following the new-scorer skill.
• Config schema in evalcore-config with sensible defaults (unit: chars, no bound = unbounded on that side).
• Inline unit tests: under min, over max, in range, and both-bounds-omitted.
• Docs line in the scorers reference.

Good first issue

This is the smallest possible end-to-end tour of the codebase — schema, scorer, factory, tests, docs. Great first PR. Comment to claim it.