fix(corpus): normalize diversify() position term so similarity can compete

The greedy score mixed incommensurate scales: cosine similarity bounded
to [-1, 1] against an absolute list index that grows with the pool. For
a candidate j positions later to be preferred at the default
diversity=0.5, its similarity advantage had to exceed j -- impossible for
the non-negative cosines real footage embeddings produce. diversify()
therefore returned the input order verbatim, placing exact-duplicate
clips in adjacent edit slots, the one thing its docstring promises to
prevent. The threshold where the knob started working also depended on
pool size (0.66 at 4 candidates, 0.95 at 11).

Normalize the position term to [0, 1] so both terms share a scale. The
documented endpoints hold exactly as before: diversity=0 returns input
order, diversity=1 picks the most mutually dissimilar. Enumerating the
position also drops the O(n^2) remaining.index() lookup per candidate.

Closes #392

tests/lib/test_corpus_diversify_scale.py

@@ -0,0 +1,126 @@
+"""Regression tests: Corpus.diversify() must actually diversify at its default.
+
+The greedy score mixed two incommensurate scales:
+
+    score = diversity * (-sim_picked) + (1 - diversity) * (-remaining.index(i))
+
+`sim_picked` is a cosine in [-1, 1]; `remaining.index(i)` is an absolute
+list position, unbounded. At the default diversity=0.5 a candidate one
+slot later needed a similarity gap > 1.0 to be preferred — impossible for
+non-negative cosines — so diversify() returned the input order verbatim,
+even when the list held exact duplicates. Its one documented job ("make
+sure no two adjacent slots are visually redundant") never happened, and
+the effective meaning of `diversity` changed with pool size.
+
+The fix normalizes the position term to [0, 1] so both terms share a
+scale. diversity=0 still returns input order; diversity=1 still picks the
+most mutually dissimilar.
+"""
+
+import sys
+from pathlib import Path
+
+import numpy as np
+import pytest
+
+PROJECT_ROOT = Path(__file__).resolve().parent.parent.parent
+sys.path.insert(0, str(PROJECT_ROOT))
+
+from lib.corpus import EMBED_DIM, ClipRecord, Corpus
+
+
+def _corpus(vectors: list[list[float]]) -> Corpus:
+    """Build an in-memory corpus from small L2-normalised vectors."""
+    corp = Corpus(Path("/nonexistent"))
+    emb = np.zeros((len(vectors), EMBED_DIM), dtype=np.float32)
+    for row, vec in enumerate(vectors):
+        emb[row, : len(vec)] = vec
+        emb[row] /= np.linalg.norm(emb[row])
+    corp.clip_embeddings = emb
+    corp.records = [
+        ClipRecord(
+            clip_id=f"c{row}",
+            source="test",
+            source_id=str(row),
+            source_url="",
+            local_path="",
+        )
+        for row in range(len(vectors))
+    ]
+    corp._id_to_row = {r.clip_id: i for i, r in enumerate(corp.records)}
+    return corp
+
+
+def test_default_diversity_separates_exact_duplicates():
+    # c1 is an exact duplicate of c0; c2/c3 are orthogonal to both.
+    corp = _corpus([[1, 0, 0], [1, 0, 0], [0, 1, 0], [0, 0, 1]])
+
+    kept = corp.diversify(["c0", "c1", "c2", "c3"], n=3)
+
+    # The duplicate must not sit adjacent to its twin at the default.
+    assert kept[0] == "c0"
+    assert kept[1] != "c1"
+
+
+def test_default_diversity_is_not_a_noop():
+    # Three exact duplicates (c0, c2, c5) interleaved with distinct clips.
+    # The old scoring returned the input order verbatim here.
+    corp = _corpus(
+        [
+            [1, 0, 0],
+            [0.8, 0.6, 0],
+            [1, 0, 0],
+            [0.6, 0.8, 0],
+            [0, 1, 0],
+            [1, 0, 0],
+        ]
+    )
+    ids = [f"c{i}" for i in range(6)]
+
+    kept = corp.diversify(ids, n=6)
+
+    assert kept != ids  # reorders when the input holds duplicates
+    for a, b in zip(kept, kept[1:]):  # and no two adjacent slots are twins
+        va = corp.clip_embeddings[corp._id_to_row[a]]
+        vb = corp.clip_embeddings[corp._id_to_row[b]]
+        assert float(va @ vb) < 0.999
+
+
+def test_diversity_semantics_do_not_collapse_with_pool_size():
+    # Duplicate of c0 right at position 1, moderately-related fillers,
+    # one orthogonal clip at the end. With the unnormalised position
+    # term the duplicate always won slot 2 at diversity=0.5 because the
+    # positional penalty scaled with the pool.
+    half_sqrt3 = np.sqrt(3) / 2
+    vectors = [[1, 0, 0], [1, 0, 0]] + [[0.5, half_sqrt3, 0]] * 8 + [[0, 0, 1]]
+    corp = _corpus(vectors)
+    ids = [f"c{i}" for i in range(len(vectors))]
+
+    kept = corp.diversify(ids, n=3, diversity=0.5)
+
+    assert kept[0] == "c0"
+    assert "c1" not in kept[:2]  # the exact duplicate no longer wins slot 2
+
+
+def test_diversity_zero_keeps_input_order():
+    corp = _corpus([[1, 0, 0], [1, 0, 0], [0, 1, 0], [0, 0, 1]])
+    ids = ["c0", "c1", "c2", "c3"]
+
+    assert corp.diversify(ids, n=4, diversity=0.0) == ids
+
+
+def test_diversity_one_picks_most_dissimilar():
+    # c1 duplicates c0; c2 and c3 are mutually orthogonal.
+    corp = _corpus([[1, 0, 0], [1, 0, 0], [0, 1, 0], [0, 0, 1]])
+
+    kept = corp.diversify(["c0", "c1", "c2", "c3"], n=3, diversity=1.0)
+
+    assert kept == ["c0", "c2", "c3"]
+
+
+def test_empty_and_unknown_ids():
+    corp = _corpus([[1, 0, 0]])
+
+    assert corp.diversify([], n=3) == []
+    assert corp.diversify(["nope"], n=3) == []
+    assert corp.diversify(["c0", "nope"], n=3) == ["c0"]