Task: Implement currency_pair and currency_pair_convention via ores.codegen.entity

Model currency_pair (identity), currency_pair_convention (1:1 quoting/date conventions), and their two new aux types (currency_pair_classification, currency_group) plus the currency_currency_group junction, via ores.codegen.entity=/ =ores.codegen.junction, per the settled field design. Generate and reconcile SQL, C++ core, messaging, and Qt profiles until the component builds cleanly. CLI/shell profiles are a separate, pre-existing codegen gap (tracked elsewhere), not blocking this task.

• [X] currency_pair, currency_pair_convention, currency_pair_classification, currency_group, and currency_currency_group_junction modelled as ores.codegen.entity=/=ores.codegen.junction.
• [X] SQL, C++ core (domain/generator/repository/service), messaging (protocol/nats-eventing/nats-handler), and Qt profiles generated for all 5.
• [X] currency_pair's base/quote/settlement currency fields use the new flagged_combo Qt facet (flag icons via FlagIconHelper).
• [X] currency_pair's list column shows a composited base+quote flag icon via the existing generic icon_columns facet.
• [X] ores.refdata.api.lib, ores.refdata.core.lib, ores.qt.refdata.lib, and both test targets build cleanly.
• [ ] compass db recreate run to verify the generated SQL applies cleanly end-to-end (not yet done this session).

Read the settled field design from the three prior design tasks (entity model, currency-group replacement, identity/convention split), then diffed the target shape against currency.org's own knobs (change- reason variability, Qt has_change_reason_cache, table display, insert-trigger validations table) to make sure nothing "standard" was missing, per explicit instruction mid-task.

Discovered the desired UX (flag-icon currency pickers) had no working codegen path: is_dynamic_combo exists in core.py but was never wired to the actual async fetch helper (populateDynamicCombo<Entity>, hand-invoked only in CurrencyDetailDialog.cpp); the real working precedent for flag-icon currency pickers (FlagIconHelper::apply_flag_icons + fetch_currency_codes) is 100% hand-written across ~10 trading forms. Per project convention (paste blocks only for genuinely one-off cases), added a new generic Qt facet, flagged_combo, to core.py + the three detail-dialog templates (header=/=ui=/=impl .org sources, tangled to .mustache) rather than a paste block, since the pattern already existed identically elsewhere. currency_pair's list-column composited flag icon needed no new work — icon_columns already supported the two-field case generically (its own docstring cites "e.g. a currency pair" as the motivating example).

Generation surfaced two pre-existing core.py bugs, both fixed: needs_counter read the primary key's derived is_text flag before it was computed (always False), and was scoped inside an if 'natural_keys' in domain_entity block that currency_pair (no natural keys) never entered — so a text-PK entity with a custom generator and no natural keys never got its uniqueness-suffix counter declared, a compile error. Also found protocol.hpp (a hand-maintained per-component aggregator of every entity's own protocol header) needed new =#include=s added by hand for the 4 new entities — codegen doesn't touch that file.

Modelling decisions of note: classification and fixing_source stayed free-text line-edits rather than dynamic combos — no working codegen path exists yet for aux-type reference-data combos either (separate, already-captured gap, see codegen_flagged_dynamic_combo.org). Dropped an initial points_factor() computed-method design (no paste point exists on the generated domain struct, only repository/service) in favour of deferring the reciprocal computation to the future ORE mapper, which can just inline 1.0 / pip_factor.

Two follow-ups deliberately left out of scope for this task:

• The composited pair-flag list column reads correctly-sized only if the generated CurrencyPairMdiWindow's list view calls setIconSize(currency_pair_icon_size()) (see FlagIconHelper.hpp) — no codegen facet sets this yet; FX Spot Grid does it by hand today. Not done here; the column will render with Qt's default (squarish) icon size until addressed.
• compass db recreate hasn't been run this session to verify the generated SQL create/drop scripts apply cleanly against a live database — build-level compilation was verified, not the DB schema itself.