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.

tests/providers/test_fidelity_planviewer.py

@@ -7,13 +7,14 @@ from pathlib import Path
 
 import pytest
 
-from broker_sync.models import Account, AccountType
+from broker_sync.models import Account, AccountType, ActivityType
 from broker_sync.providers.fidelity_planviewer import (
     ACCOUNT_ID,
     FidelityCreds,
     FidelityPlanViewerProvider,
     FidelityProviderConfigError,
     fidelity_holdings_to_snapshot,
+    gains_offset_delta_activity,
 )
 from broker_sync.providers.parsers.fidelity import (
     parse_transactions_html,
@@ -146,3 +147,63 @@ def test_provider_caches_holdings_for_cli_snapshot_push() -> None:
     # Pre-fetch state: empty
     assert prov.last_holdings == []
     assert prov.last_total_contribution == Decimal(0)
+
+
+# -- delta-shaped gains offset (the monthly accumulation mechanism) --
+
+
+def _holdings_summing_to(total: Decimal) -> list:
+    from broker_sync.providers.parsers.fidelity import FidelityHolding
+    return [FidelityHolding(
+        fund_code="KDOA", fund_name="Test", units=Decimal("100"),
+        unit_price=total / Decimal("100"), currency="GBP", total_value=total,
+        units_by_source={},
+    )]
+
+
+def test_gains_delta_emits_deposit_when_gain_exceeds_prior_offset() -> None:
+    # pot £145k, real contrib £102k → current gain £43k; prior offset £35k
+    # → delta = +£8k
+    activity = gains_offset_delta_activity(
+        holdings=_holdings_summing_to(Decimal("145000")),
+        total_real_contribution=Decimal("102000"),
+        prior_offset_cumulative=Decimal("35000"),
+        as_of=datetime(2026, 5, 17, tzinfo=UTC),
+    )
+    assert activity is not None
+    assert activity.activity_type == ActivityType.DEPOSIT
+    assert activity.amount == Decimal("8000")
+    assert activity.external_id == "fidelity:gains-delta:2026-05-17"
+    assert "unrealised-gains-offset" in (activity.notes or "")
+
+
+def test_gains_delta_emits_withdrawal_on_market_drop() -> None:
+    # pot dropped: current gain £30k, prior offset £35k → delta = -£5k
+    activity = gains_offset_delta_activity(
+        holdings=_holdings_summing_to(Decimal("132000")),
+        total_real_contribution=Decimal("102000"),
+        prior_offset_cumulative=Decimal("35000"),
+        as_of=datetime(2026, 5, 17, tzinfo=UTC),
+    )
+    assert activity is not None
+    assert activity.activity_type == ActivityType.WITHDRAWAL
+    assert activity.amount == Decimal("5000")
+
+
+def test_gains_delta_suppressed_below_minimum() -> None:
+    # delta ~£0.20, below the £0.50 min — skip emission to avoid noise.
+    activity = gains_offset_delta_activity(
+        holdings=_holdings_summing_to(Decimal("137000.20")),
+        total_real_contribution=Decimal("102000"),
+        prior_offset_cumulative=Decimal("35000"),
+        as_of=datetime(2026, 5, 17, tzinfo=UTC),
+    )
+    assert activity is None
+
+
+def test_gains_delta_none_when_no_holdings() -> None:
+    assert gains_offset_delta_activity(
+        holdings=[], total_real_contribution=Decimal("0"),
+        prior_offset_cumulative=Decimal("0"),
+        as_of=datetime(2026, 5, 17, tzinfo=UTC),
+    ) is None