feat(provenance): layered ProvenanceEnvelope + effective() leaf (WP-0007 T1)

Dependency-free leaf rail: ProvenanceEnvelope + SpanProvenanceDelta + effective()
(page envelope ⊕ span delta; zero-cost inheritance when uniform). Liveness/
Staleness/OverlayState enums. 6 tests green. (blueprint §7.3, §11)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_provenance.py

@@ -0,0 +1,73 @@
+"""Tests for the provenance leaf rail (SHARD-WP-0007 T1)."""
+
+from datetime import datetime, timezone
+
+from shard_wiki.provenance import (
+    Liveness,
+    OverlayState,
+    ProvenanceEnvelope,
+    SpanProvenanceDelta,
+    Staleness,
+    effective,
+)
+
+
+def _base() -> ProvenanceEnvelope:
+    return ProvenanceEnvelope(
+        source_shard="shardA",
+        liveness=Liveness.STATIC,
+        staleness=Staleness.FRESH,
+        source_rev="r1",
+        observed_at=datetime(2026, 6, 15, tzinfo=timezone.utc),
+    )
+
+
+def test_no_delta_returns_base_identity():
+    base = _base()
+    assert effective(base) is base
+    assert effective(base, SpanProvenanceDelta()) is base
+
+
+def test_empty_delta_is_detected():
+    assert SpanProvenanceDelta().is_empty()
+    assert not SpanProvenanceDelta(source_shard="other").is_empty()
+
+
+def test_single_field_override():
+    base = _base()
+    eff = effective(base, SpanProvenanceDelta(source_shard="shardB"))
+    assert eff.source_shard == "shardB"
+    # Untouched fields inherit from the page envelope.
+    assert eff.source_rev == "r1"
+    assert eff.liveness is Liveness.STATIC
+    # Base is unchanged (frozen, pure composition).
+    assert base.source_shard == "shardA"
+
+
+def test_partial_delta_overrides_several_fields():
+    base = _base()
+    eff = effective(
+        base,
+        SpanProvenanceDelta(
+            staleness=Staleness.STALE,
+            overlay_state=OverlayState.DRAFT,
+            divergence=("shardB:Home",),
+        ),
+    )
+    assert eff.staleness is Staleness.STALE
+    assert eff.overlay_state is OverlayState.DRAFT
+    assert eff.divergence == ("shardB:Home",)
+    assert eff.source_shard == "shardA"  # inherited
+
+
+def test_divergence_empty_tuple_overrides_but_none_inherits():
+    base = ProvenanceEnvelope(source_shard="s", divergence=("x",))
+    # None → inherit the page's divergence.
+    assert effective(base, SpanProvenanceDelta(divergence=None)).divergence == ("x",)
+    # Explicit empty tuple → override (clear it).
+    assert effective(base, SpanProvenanceDelta(divergence=())).divergence == ()
+
+
+def test_envelope_is_hashable_and_value_equal():
+    assert _base() == _base()
+    assert len({_base(), _base()}) == 1