[guijacquemet]

What and why

Implements the owner's decision on issue #58: a new streaming API, contiguity required (refused otherwise), eager API unchanged.

• iter_image_summaries(records, percentile) is a generator. It holds one image's planes at a time, yields that image's ImageSummary when the next image starts, and drops the histograms. Each summary equals the eager one (same _summary).
• Precondition: all Z/T rows of an image are contiguous. Rows are never sorted or staged. Refused with ValueError: an image whose rows are not together, a source file under two label sets, repeated plane_id, and different channel counts.
• Summaries come in first-appearance order, which equals the eager order for rows sorted by condition, repeat and path.
• Error contract (documented in the docstring, contract §7.1 and CHANGELOG): channel counts are only compared as images are read, so an error can follow summaries already yielded. A run that raised is incomplete and its output must be discarded.
• image_summaries behaviour is unchanged. Only the percentile check moved into a shared helper.
• No benchmark was run, so no performance claim is made.

Regression tests (tests/test_summaries.py)

• test_streaming_summaries_equal_the_eager_ones, test_streaming_emits_one_image_at_a_time_and_drops_its_histograms, test_streaming_refuses_rows_that_are_not_together, and the other test_streaming_* tests.
• On old main source the test module fails at collection: ImportError, because iter_image_summaries does not exist. All pass on the new code.

Checks

python tools/check.py: 720 passed, 2 skipped (tests/test_rgb.py:80, optional csbdeep not installed; not claimed as passed), 4 warnings (pre-existing, in test_pooling.py). The code-rules and ruff step is clean.

Part of #58? Fully implements the decided scope (streaming API plus refusal), so: Fixes #58 for the decided scope. The issue's benchmarks and external-sort mode were not part of the decision.

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