A pathological `ChunkPolicy { overlap_tokens >= target_tokens }`
caused `md-heading-v1` to degenerate into 1-block-per-chunk: the
seeded `acc.text_tokens` already exceeded `target_tokens` before any
fresh content landed, so the next paragraph immediately tripped the
`would_exceed` flush.
Clamp the seed budget in `collect_overlap_seed` to
`min(overlap_tokens, target_tokens / 2)`. This guarantees that after
seeding, the chunk has at least `target/2` worth of room for new
content before flushing, restoring the intended paragraph-overlap
behavior on every reasonable and unreasonable policy.
Adds a regression test pinning a 50/200 (overlap = 4× target) policy
and asserting every emitted chunk holds ≥2 blocks.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
altair823
a81460f9d0