Fix Depot Transfer Cost Basis

[ma4nn]

Summary

This is a follow-up PR for #59 to separate the multiple changes in that PR into single ones.

It teaches the FIFO cost-basis engine to respect Portfolio Performance depot transfers. On a linked TRANSFER_OUT/TRANSFER_IN pair, the cost-basis lot is relocated from the source to the destination account, preserving original acquisition date, price, and (prorated) fees — so per-account cost basis, view securities, and simulate share-sell stay correct after transfers.

• Engine (cost_basis.py): relocate lots on linked transfers; split + prorate on partial transfers; destination
  consumes by original acquisition date; FIFO scoped per (account, security).
• Link (pp_portfolio_builder.py, schemas.py): destination read from PP's xact_cross_entry, surfaced as
  transferTargetAccount.
• Vorabpauschale (tax.py): deemed income attributed per lot, fixing a double-count / crash when a transfer creates
  two lots sharing one (date, account, security) key.
• share-sell (simulate_share_sell.py): --account-id scopes by net share balance; cross-account FIFO resolves
  transferred lots.
• Validation (rules.py): built-in unlinked-depot-transfer rule (a warning, on by default) flags transfers with
  no cross-entry link.

Behavior changes

• share-sell --shares without --account-id now picks the globally-oldest lot across accounts (was per-account).
• share-sell --account-id <source> no longer offers an unlinked transfer's net-0 source lots; cost-basis totals stay
  correct, and the data is surfaced by the default-on validation rule plus an engine WARNING during computation.

Known limitations

• Same-day ordering is ambiguous (PP dates are day-granular) — resolved deterministically but a same-day cross-account transfer may not match PP's attribution; shares are always conserved. Same cause can strand shares in a chained same-day A → B → C (logs a warning).
• Unlinked transfers: cost basis kept in source (flagged by the default-on unlinked-depot-transfer warning and an engine log warning), not sellable via --account-id.
• A stale pre-feature .cache.db can OperationalError on the new join (low probability; delete cache or use --no-cache).

Testing

New tests cover full/partial/chained/round-trip/over transfers, cross-account FIFO by acquisition date, per-lot
Vorabpauschale, share-sell scoping, per-security FIFO isolation, and the validation rule. Full suite passes; pylint 10/10;
bandit clean.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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