Derive net and growth metrics from stored atoms (computed)

[Gorm2303]

Given Tier 1 stores only atoms
When producing Tier 1 results
Then backend derives key computed metrics consistently from atoms:

Per year:

net_year = return_year - tax_year - fee_year

To-date:

principal_to_date = Σ deposit_year

growth_to_date = capital_end - principal_to_date (timing convention must be explicit)

Acceptance criteria

Derived metrics are computed from Tier 1 atoms only

Timing conventions are explicit and documented (end-of-year snapshot alignment)

Derived metrics are tagged COMPUTED in glossary/taxonomy