* fix(simplefin): treat Vanguard/Fidelity cost_basis as total when needed
PR #1692 normalized SimpleFIN holdings cost_basis under the assumption
that the `cost_basis` / `basis` keys carry a per-share value (per the
SimpleFIN spec) and only `total_cost` / `value` carry a total position
cost. Vanguard and Fidelity violate the spec — they populate
`cost_basis` with the *total* (see the payload in #1182). After PR
#1692 those holdings get stored with cost_basis = total, and
Holding#calculate_trend then computes previous = qty × avg_cost, so the
"previous" value is inflated by a factor of qty and an entire
investment account renders a phantom return of roughly -(1 − 1/qty),
i.e. -97% to -99%.
Fix: sanity-check raw cost_basis against the holding's market share
price. Let share_price = market_value / qty; the geometric midpoint
between "raw is per-share" (raw ≈ share_price) and "raw is total"
(raw ≈ qty × share_price) is share_price × √qty. If raw is above the
midpoint it is divided by qty; otherwise it is kept as per-share.
Falls back to the pre-fix behaviour (trust the spec) when market_value
or qty is unavailable, so confidently-correct readings are never made
worse.
Verified against the reported Vanguard payload (qty=139, cost_basis=
22004.40, market_value=22626.42): normalize_cost_basis now returns
$158.31/share, matching 22004.40 / 139, and the phantom -99% return
collapses to a realistic ~+2.8%. Per-share readings ($45 cost on a $50
share price) remain untouched.
Closes#1718. Refs #1182, #1692.
* fixup: replace cost_basis heuristic with institution allowlist
Codex and @EdeAbreu23 flagged a real false-positive in the previous
geometric-midpoint heuristic: a legitimate per-share `cost_basis` on a
holding with a large unrealized loss (e.g. 100 shares with $100/share
basis now worth $5/share) trips `share_price × √qty` and gets divided
to $1/share — corrupting any standards-compliant brokerage with a big
loss.
Adopt @EdeAbreu23's safer shape:
- total_cost / value: always divide by qty (unchanged from #1692).
- cost_basis / basis: keep as-is by default.
- Only divide cost_basis / basis when the holding's SimpleFIN account
is connected to a known-misbehaving institution. Allowlist starts
with `vanguard` and `fidelity`, matched case-insensitively against
the account's stored org name and domain. Easy to extend as more
brokerages turn up.
Trades a small maintenance cost (curated list) for zero risk of
corrupting compliant providers.
Verified against five scenarios (all expected):
Vanguard total in cost_basis (allowlist) → +2.83%
Fidelity total in basis (allowlist) → +33.33%
Big-loss per-share (Codex case) → -95.0% (preserved)
Honest per-share, small loss → +11.11% (unchanged)
total_cost on any institution → +11.11% (unchanged)
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Co-authored-by: plind-junior <plind-junior@users.noreply.github.com>
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