Handle Spin-off events as buys (#35)

Reviewed-on: #35
Co-authored-by: Natercio Moniz <[email protected]>

internal/trading212/README.md

@@ -0,0 +1,50 @@
+# Trading212
+
+Turns Trading212 account statements (CSV) into the internal record model used
+to build Anexo J reports. This file records how statement events are
+interpreted, and why.
+
+## Spin-off events
+
+A spin-off distributes shares of one company to the shareholders of another.
+Trading212 reports it as an `Action` of `Spin off`:
+
+```
+Spin off,2026-02-11 13:41:43+00:00,US9418481035,WAT,"Waters",EOF46451921527,0.0406450000,0E-10,USD,,,,0.00,"EUR",,,,,,
+```
+
+These rows are booked as **buys with a cost of zero**: quantity and timestamp
+are kept, and every cost column (price, result, total, stamp duty reserve tax,
+currency conversion fee, French transaction tax) is required to be zero —
+anything non-zero, or a non-positive quantity, is logged and returned as an
+error, stopping the run. A row that surprises the reader must be looked at by
+a human, not silently booked.
+
+### Why cost zero is fair
+
+No money was paid for the new shares, and their value came out of the value
+of the parent shares — on the distribution date the parent's price drops by
+roughly the distributed value. The position's total basis is unchanged; it is
+just partitioned over two lots: the spin-off lot at `0` and the parent lot at
+its original, unreduced basis. Selling the spin-off lot books full proceeds as
+gain; selling the parent lot shows the offsetting smaller gain or bigger loss.
+Netted together, the result equals the true economic gain.
+
+Example — buy 100 parent shares for 10,000; a spin-off delivers new shares
+worth 2,000; later sell the new shares for 2,200 and the parent for 8,100:
+
+| Lot    | Basis (cost 0) | Sold for | Gain (cost 0) | Gain (pro-rata) |
+| ------ | -------------: | -------: | ------------: | --------------: |
+| New    | 0              | 2,200    | 2,200         | 200             |
+| Parent | 10,000         | 8,100    | -1,900        | 100             |
+| Total  |                |          | **300**       | **300**         |
+
+Either partition of the same 10,000 basis yields the same net gain of 300
+(true gain: 10,300 received − 10,000 paid) — as long as gains and losses are
+assessed together, as they are in Anexo J within the same year. If the two
+lots are sold in different years the per-year attribution differs (the total
+does not). Trading212 statements don't report a pro-rata basis allocation, so
+cost 0 also keeps the report reproducible from the statement alone.
+
+> [!WARNING]
+> Verify any outputs of this tool on your own or with a certified accountant.