feat(ledger): K1 — savings ledger + goal contract (parity rows 2-3)

New ledger.py: typed SavingsLedger; realize is fail-closed on an APPROVED
FeedbackContract + a named expert + an explicit timestamp (the §6 determinism
rule — no wall-clock default). The sum key is DIMENSION-FREE (the dimension
label is annotation only and never participates in the mint), so the same
realized saving surfaced via two dimensions lands in one first-write-wins
slot and is never double-counted. Deterministic JSON persistence
(sort_keys, indent 2, LF, trailing newline), schema-validated on load.

New goals.py: GoalContract (absolute target, hard/soft, fail-fast §10).
A hard goal reached raises GoalReached, a structured stop event carrying
target + observed — never a silent stop; soft flags without stopping.
The percent-goal baseline is D-E-gated: the field is reserved and
construction refuses with an explicit NotImplementedError.

Semantics are marked STACK-LOCAL in the docstrings — mirrored from the MAF
plan's capability description, never from MAF code; format shareability
stays a proposed decision point in the brief.

Two detach proofs delivered (decision gate removed -> red; dimension into
the key mint -> the double-counting test red). 400 -> 426 tests; README
synced (test count + a Value layer module block).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/portfolio_optimiser_claude/ledger.py

@@ -0,0 +1,146 @@
+"""The savings ledger (parity rows 2-3) — typed, fail-closed, dimension-free sum.
+
+Realized savings enter the book ONLY through the expert gate: ``realize``
+requires an explicit APPROVED expert verdict (the §4.1 binary run-path shape),
+a named expert identity, and an explicit timestamp (no wall-clock default —
+the same determinism rule as promotion §6). Entries are keyed DIMENSION-FREE
+(project + candidate identity + amount; the dimension label is annotation
+only), so the same realized saving surfaced via two dimensions lands in one
+first-write-wins slot (§4.2-style idempotence) and is never double-counted.
+Persistence is deterministic JSON: sort_keys, indent 2, LF, trailing newline.
+
+Ledger semantics are a STACK-LOCAL contract mirrored from the MAF plan's
+capability description — never from MAF code; detail semantics may diverge
+(shareability of the format is a proposed decision point in the brief).
+"""
+
+from __future__ import annotations
+
+import hashlib
+import json
+from collections.abc import Iterable
+from pathlib import Path
+
+from pydantic import BaseModel, ConfigDict, Field
+
+from portfolio_optimiser_claude.contracts import FeedbackContract
+
+
+class LedgerGateError(ValueError):
+    """A realization was refused at the expert gate — nothing enters the book."""
+
+
+class LedgerEntry(BaseModel):
+    """One realized saving: dimension-free key, provenance-stamped (expert + timestamp)."""
+
+    model_config = ConfigDict(frozen=True)
+
+    key: str = Field(min_length=1)
+    project: str = Field(min_length=1)
+    measure_type: str = Field(min_length=1)
+    affected_codes: tuple[str, ...]
+    amount_nok: float = Field(gt=0, allow_inf_nan=False)
+    expert: str = Field(min_length=1)
+    timestamp: str = Field(min_length=1)
+    dimension: str | None = None
+
+
+class _LedgerFile(BaseModel):
+    """The persisted ledger shape — schema-validated on load, fail-fast (§10-style)."""
+
+    entries: list[LedgerEntry]
+
+
+def _mint_entry_key(
+    project: str, measure_type: str, affected_codes: tuple[str, ...], amount_nok: float
+) -> str:
+    # DIMENSION-FREE by construction: the dimension label never participates,
+    # so two dimensions surfacing the same realized saving mint the same key.
+    canonical = json.dumps(
+        {
+            "affected_codes": sorted(affected_codes),
+            "amount_nok": amount_nok,
+            "measure_type": measure_type,
+            "project": project,
+        },
+        sort_keys=True,
+        separators=(",", ":"),
+    )
+    return hashlib.sha256(canonical.encode("utf-8")).hexdigest()[:16]
+
+
+class SavingsLedger:
+    """The typed book of realized savings — FIRST-write-wins per dimension-free key."""
+
+    def __init__(self) -> None:
+        self._entries: dict[str, LedgerEntry] = {}
+
+    def __len__(self) -> int:
+        return len(self._entries)
+
+    def realize(
+        self,
+        *,
+        project: str,
+        measure_type: str,
+        affected_codes: Iterable[str],
+        amount_nok: float,
+        verdict: FeedbackContract,
+        expert: str,
+        timestamp: str,
+        dimension: str | None = None,
+    ) -> LedgerEntry:
+        """Enter one realized saving — fail-closed on anything short of expert approval.
+
+        ``expert`` and ``timestamp`` are explicit REQUIRED arguments (no
+        wall-clock default, §6-style determinism). Re-realizing the same
+        dimension-free key returns the FIRST entry unchanged (idempotent).
+        """
+        if verdict.decision != "approved":
+            raise LedgerGateError(
+                f"realization refused: verdict decision {verdict.decision!r} is not "
+                "'approved' — only expert-approved savings enter the book (fail-closed)"
+            )
+        if not expert.strip():
+            raise LedgerGateError(
+                "realization refused: expert identity is blank — realized savings "
+                "require a named expert (fail-closed)"
+            )
+        codes = tuple(sorted(affected_codes))
+        entry = LedgerEntry(
+            key=_mint_entry_key(project, measure_type, codes, amount_nok),
+            project=project,
+            measure_type=measure_type,
+            affected_codes=codes,
+            amount_nok=amount_nok,
+            expert=expert,
+            timestamp=timestamp,
+            dimension=dimension,
+        )
+        return self._entries.setdefault(entry.key, entry)
+
+    def entries(self) -> list[LedgerEntry]:
+        """All entries, ordered by key (deterministic)."""
+        return sorted(self._entries.values(), key=lambda entry: entry.key)
+
+    def total_realized_nok(self) -> float:
+        """The dimension-free sum: one addend per distinct key, never double-counted."""
+        return sum(entry.amount_nok for entry in self._entries.values())
+
+    def to_json(self) -> str:
+        """Deterministic JSON: sort_keys, indent 2, trailing newline."""
+        payload = {"entries": [entry.model_dump() for entry in self.entries()]}
+        return json.dumps(payload, sort_keys=True, indent=2, ensure_ascii=False) + "\n"
+
+    def save(self, path: Path) -> None:
+        """Persist deterministically (LF only) — identical books yield identical bytes."""
+        path.write_text(self.to_json(), encoding="utf-8", newline="\n")
+
+    @classmethod
+    def load(cls, path: Path) -> SavingsLedger:
+        """Load a persisted ledger, schema-validated fail-fast (§10-style)."""
+        parsed = _LedgerFile(**json.loads(path.read_text(encoding="utf-8")))
+        ledger = cls()
+        for entry in parsed.entries:
+            ledger._entries.setdefault(entry.key, entry)
+        return ledger