Story: Hotfix: Windows link fails on missing IFeed ctor/dtor exports

Restore the Windows CI build: make ores.marketdata.api.dll export IFeed's default ctor and virtual dtor so consumers (ores.synthetic.api feeds) can import them.

• The Windows CI build links ores.synthetic.api.dll in all three configs.
• IFeed remains abstract and header-declarable; consumers unchanged.

• Root cause: IFeed is ORES_MARKETDATA_API_EXPORT (dllimport in consumers) with inline-defaulted default ctor and virtual dtor. The class is abstract and never constructed or destroyed inside the producer, so the producer never emits or exports the special members; consumers (fx_spot_feed, ir_curve_feed) import them, and Windows fails at the link of ores.synthetic.api.dll.
• Fix: define both special members out-of-line in the producer (domain/i_feed.cpp) rather than in the header — the class-level export macro exports them from ores.marketdata.api.dll. Consumers unchanged.
• IPlugin (ores.qt) has the same latent shape (abstract, exported, inline-defaulted virtual dtor); the in-tree build is unaffected (the producer odr-uses the ctor via PluginBase.cpp), so it is tracked as a follow-up, not part of this hotfix.

Implemented and merged as PR #1988 (scaffold PR #1987 merged first). IFeed's default ctor and virtual dtor are defined out-of-line in the producer (domain/i_feed.cpp), so the class-level export macro exports them from ores.marketdata.api.dll; the undefined-symbol link failure of ores.synthetic.api.dll (red for 8 consecutive Windows runs) is healed in all three configs. Linux build and tests pass; the scheduled Continuous Windows run after merge is the Windows-side confirmation.