Trading module now VN stocks only with dynamic symbol resolution.
Update test counts (105), remove crypto/gold/forex references from
project-level docs, update architecture file tree descriptions.
Extract trading module details (commands, data model, price APIs, file
layout) from architecture.md and codebase-summary.md into
src/modules/trading/README.md. Project-level docs now only contain
global framework info with pointers to module-local READMEs.
Add trading module section (§13) to architecture.md covering commands,
data model, price sources, and file layout. Update file trees, test
counts (56→110), and module registry snippet in both docs.
