fix(producer): drop empty trailing chunk slice in distributed render plan (#1133)

resolveChunkPlan caps chunkCount at maxParallelChunks from the naive
count, then rounds effectiveChunkSize up to ceil(totalFrames /
chunkCount). When that ceil rounds up, the first (chunkCount - 1) chunks
can already cover every frame, so buildChunkSlices emits a final slice
with startFrame >= totalFrames — an empty [n, n) or inverted range.
renderChunk rejects it (framesInChunk <= 0) and, under Step Functions
retries, fails the whole distributed render even though [0, totalFrames)
is fully covered.

This is reachable from the user-facing CLI: `hyperframes lambda render
--chunk-size 10 --max-parallel-chunks 12` on a ~4s/30fps (121-frame)
composition yields chunkCount=12, effectiveChunkSize=11, and a 12th slice
of [121, 121).

Tighten chunkCount to ceil(totalFrames / effectiveChunkSize) after the
size is finalized, so the union stays exactly [0, totalFrames) with no
empty tail. This only lowers chunkCount in the explicit-small-chunkSize
case; the auto-sized and large-chunkSize paths already satisfy
ceil(totalFrames / effectiveChunkSize) >= chunkCount, so it's a no-op
there (existing tests' chunkCount values are unchanged).

Adds a regression test for the 121/10/12 case plus a grid property test
asserting contiguous, non-empty, exact coverage across explicit sizes.

Co-authored-by: Carlos Alcaraz <193642530+calcarazgre646@users.noreply.github.com>