feat(fase1): dimension-free dedup key + cross-dimension overlap flag (F1)

tests/test_ledger_loadbearing.py

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+"""Load-bearing seams for the savings ledger (Fase 1 — SC4/SC5/SC6/SC8).
+
+Step 5 (SC4) — the dimension-free dedup gate: the same underlying candidate (same codes+measure+
+amount) realized under two dimensions counts ONCE in the portfolio sum — double-counting across
+dimensions is the exact error SC4 warns against — while the overlap is FLAGGED (never silently
+merged). RED if the dimension-free sum-key dedup is detached (the sum doubles). Inverse control:
+two DISTINCT realizations with the same codes+measure but a different amount stay SEPARATE (a
+magnitude-free identity would collide them).
+
+(Step 6 extends this file with the fail-closed ``realize`` gate + øre-boundary tests; Step 8 adds
+the goal-stop detach.)
+
+Pattern: ``tests/test_step8_promotion_loadbearing.py:77`` (fail-closed gate trio).
+"""
+
+from __future__ import annotations
+
+from portfolio_optimiser.ledger import (
+    LedgerEntry,
+    SavingsLedger,
+    _candidate_identity,
+    stamp,
+)
+
+_TS = "2026-07-06T00:00:00Z"
+_CODES = frozenset({"ENERGI-TOTAL-EL"})
+_MEASURE = "energy_efficiency"
+
+
+def _identity(amount_ore: int) -> str:
+    return _candidate_identity(affected_codes=_CODES, measure_type=_MEASURE, amount_ore=amount_ore)
+
+
+def _entry(dimension: str, *, amount_ore: int) -> LedgerEntry:
+    return LedgerEntry(
+        project_id="P1",
+        dimension=dimension,
+        candidate_identity=_identity(amount_ore),
+        amount_ore=amount_ore,
+        verdict_id="v1",
+        provenance=stamp(approver="ekspert", experiment="sim", timestamp=_TS),
+    )
+
+
+def test_same_candidate_two_dimensions_counts_once_and_flags_overlap() -> None:
+    """LOAD-BEARING (SC4): the same underlying candidate realized under two dimensions contributes
+    ONCE to the portfolio sum, and the overlap is flagged. RED if the dimension-free sum-key dedup
+    is detached in ``_dedup_amount`` (the sum then doubles to 10000)."""
+    ledger = SavingsLedger()
+    assert ledger.add_realized(_entry("energi", amount_ore=5000)) is True
+    # Same candidate identity (same codes+measure+amount), a DIFFERENT dimension -> distinct full
+    # key -> stored, so the overlap is visible; the sum must still count it once.
+    assert ledger.add_realized(_entry("asfalt", amount_ore=5000)) is True
+
+    assert ledger.portfolio_total() == 5000, (
+        "double-counted the same candidate across dimensions (SC4)"
+    )
+    assert ledger.per_project_total("P1") == 5000
+    assert ledger.overlaps() == [("P1", _identity(5000))]
+
+
+def test_distinct_amounts_same_codes_measure_are_not_collided() -> None:
+    """INVERSE CONTROL (fixes the magnitude-free collision): two DISTINCT realizations with the same
+    codes+measure but a different amount are separate candidates -> both counted, no overlap."""
+    ledger = SavingsLedger()
+    assert ledger.add_realized(_entry("energi", amount_ore=5000)) is True
+    assert ledger.add_realized(_entry("energi", amount_ore=7000)) is True  # distinct identity
+
+    assert ledger.portfolio_total() == 12000  # both counted (no collision)
+    assert ledger.overlaps() == []  # single dimension -> no cross-dimension overlap
+
+
+def test_exact_full_key_duplicate_is_not_stored_twice() -> None:
+    """An exact (project, dimension, candidate) re-add is a no-op — storage keys on the full key."""
+    ledger = SavingsLedger()
+    assert ledger.add_realized(_entry("energi", amount_ore=5000)) is True
+    assert ledger.add_realized(_entry("energi", amount_ore=5000)) is False  # exact duplicate
+    assert ledger.portfolio_total() == 5000