fix(ledger): normalize every load rejection to ValueError at the ledger's own entrance

SavingsLedger.load unpacked the payload with `**`, so a valid-JSON but
non-object book ([], "x", 3, null) escaped as a raw TypeError — a failure mode
no caller catching ValueError would see. The run path was already covered:
valuereport.load_ledger caught the TypeError and re-raised it as ValueError,
and `run.py --goals` goes through that function. The leak reached only callers
outside that one path, which is why the suite stayed green.

The fix moves the normalization DOWN into ledger.py, where the public boundary
is, and deletes the now-dead patch in valuereport.load_ledger. One except
clause now covers the whole boundary: unparsable bytes (JSONDecodeError),
non-object top level (explicit check), wrong-shaped object (ValidationError).

Load-bearing (§11): the new TestLoadHasOneFailureType went RED before the fix
with exactly the TypeError it exists to forbid — pytest.raises(ValueError) does
not swallow it. Detach point named in the class docstring: drop the isinstance
check and the array/string cases raise TypeError again.

Found by cross-checking MAF's 7dab2df; queued in STATE as post 2b, approved by
the operator this session. 604 -> 612 passed, ruff + mypy --strict clean.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MQu2xxwedckjU56byu1aUG

src/portfolio_optimiser_claude/valuereport.py

@@ -292,19 +292,16 @@ def _project_value(
 def load_ledger(ledger_path: Path | None) -> SavingsLedger:
     """Load the book fail-fast (§10) — a wrong-SHAPE ledger never masquerades as empty.
 
-    ``None`` means no book was supplied (an empty one). A path that exists but
-    holds valid JSON of the wrong shape (an array, a string, ``{"entries": {}}``)
-    raises: reading it as an empty book would silently report every realized
-    saving as unmarked. ``SavingsLedger.load`` unpacks the payload, so a non-object
-    top level surfaces as ``TypeError`` — normalized here to ``ValueError`` so the
-    caller has ONE failure type to catch.
+    ``None`` means no book was supplied (an empty one); anything else is opened
+    by ``SavingsLedger.load``, which refuses malformed bytes, a non-object top
+    level and a wrong-shaped object alike as ``ValueError``. Reading any of them
+    as an empty book would silently report every realized saving as unmarked.
+    This function adds ONLY the None case — the failure-type normalization lives
+    at the ledger's own entrance, so every caller gets it, not just this path.
     """
     if ledger_path is None:
         return SavingsLedger()
-    try:
-        return SavingsLedger.load(ledger_path)
-    except TypeError as exc:  # non-object JSON: `**` needs a mapping
-        raise ValueError(f"ledger {ledger_path} is not a JSON object: {exc}") from exc
+    return SavingsLedger.load(ledger_path)
 
 
 def build_value_report(