fidelity: replace snapshot-push with delta gains-offset DEPOSITs

Per-fund snapshot import landed quantities but dropped cost basis +
needed a separate quote-push path we never identified. Snapshotting
also collided with WF's own TOTAL aggregation and ZEROED the Fidelity
cash balance.

Simpler plan: each monthly scrape emits a single DEPOSIT (or
WITHDRAWAL on a market drop) sized to the delta between the live
PlanViewer pot value and Wealthfolio's running total. dav_corrected
PG view continues to subtract these offsets from net_contribution so
the dashboard Growth/ROI math stays right.

- New gains_offset_delta_activity() — current_gain - prior_offset.
- New WealthfolioSink.cumulative_amount_with_notes_prefix() — sums
  the existing fidelity-planviewer:unrealised-gains-offset DEPOSITs
  in WF so we know what's already been emitted.
- CLI runs sync_provider_to_wealthfolio first (cash flows), then
  computes + emits the delta via import_activities.
- 4 new provider tests for the delta logic; full suite (144 + 1
  skipped) green; mypy + ruff clean.

The old fidelity_holdings_to_snapshot helper + push_manual_snapshots
sink method stay for future use but are no longer called.

broker_sync/providers/fidelity_planviewer.py

@@ -103,10 +103,10 @@ class FidelityPlanViewerProvider:
       storage_state, navigates to the transaction-history page with a wide
       date range, scrapes the table, and intercepts the valuation XHR.
     - After ``fetch()`` completes, ``last_holdings`` holds the per-fund
-      unit positions captured in the same scrape — used by the
-      ``fidelity-ingest`` CLI to push a manual snapshot to Wealthfolio
-      so per-fund quantities + cost basis land in the Positions table
-      (the activity stream alone only carries cash flows).
+      unit positions and ``last_total_contribution`` the cumulative cash
+      contribution — used by the ``fidelity-ingest`` CLI to emit a
+      delta-shaped DEPOSIT that nudges WF's net worth to match the
+      PlanViewer reported pot value (see ``gains_offset_delta_activity``).
     """
     name = "fidelity-planviewer"
 
@@ -162,12 +162,53 @@ class FidelityPlanViewerProvider:
             if before is not None and tx.date >= before:
                 continue
             yield _tx_to_activity(tx)
-        # NB: the gains-offset DEPOSIT we used to emit here is superseded
-        # by the manual snapshot push the CLI does after fetch() drains.
-        # The snapshot sets per-fund quantity + cost basis directly, so
-        # Wealthfolio computes growth from positions instead of needing a
-        # fake cash entry. Old offset rows still in WF are corrected at
-        # the dashboard layer by the dav_corrected view.
+        # Gains-offset DEPOSITs are emitted by the CLI (which has the
+        # prior cumulative offset from WF). See `gains_offset_delta_activity`.
+
+
+def gains_offset_delta_activity(
+    holdings: list[FidelityHolding],
+    total_real_contribution: Decimal,
+    prior_offset_cumulative: Decimal,
+    as_of: datetime,
+    min_delta: Decimal = Decimal("0.5"),
+) -> Activity | None:
+    """Compute the gains-offset DELTA since the last scrape and shape it
+    as a DEPOSIT (or WITHDRAWAL on a market drop).
+
+    The pension's per-fund prices aren't trackable in WF directly (no
+    public quote feed for these institutional life-fund share classes).
+    Instead, each monthly scrape emits a single small DEPOSIT/WITHDRAWAL
+    sized to ``(current_pot - real_contributions) - prior_cumulative_offset``
+    — i.e., the growth (or loss) accrued since the last run.
+
+    Wealthfolio's net_contribution then incorrectly includes all these
+    offsets; the ``dav_corrected`` PG view subtracts them back out so the
+    dashboard's Growth/ROI panels remain accurate. The deterministic
+    external_id (per scrape date) lets re-runs of the same day overwrite
+    rather than stack duplicates.
+    """
+    if not holdings:
+        return None
+    current_pot = sum((h.total_value for h in holdings), Decimal(0))
+    current_gain = current_pot - total_real_contribution
+    delta = current_gain - prior_offset_cumulative
+    if abs(delta) < min_delta:
+        return None
+    return Activity(
+        external_id=f"fidelity:gains-delta:{as_of.date().isoformat()}",
+        account_id=ACCOUNT_ID,
+        account_type=AccountType.WORKPLACE_PENSION,
+        date=as_of,
+        activity_type=ActivityType.DEPOSIT if delta > 0 else ActivityType.WITHDRAWAL,
+        currency=_CCY,
+        amount=abs(delta),
+        notes=(
+            f"fidelity-planviewer:unrealised-gains-offset delta=£{delta} "
+            f"(pot=£{current_pot}, contrib=£{total_real_contribution}, "
+            f"prior_offset=£{prior_offset_cumulative})"
+        ),
+    )
 
 
 def fidelity_holdings_to_snapshot(