refactor: standardize imports/exports with barrel pattern (Closes #324)

[Carlys17]

Summary

Standardize import/export patterns across the SDK using barrel files.

• Created src/ARCHITECTURE.md documenting barrel-export convention
• Added barrel files: errors/index.ts, contracts/index.ts, types/index.ts, utils/index.ts
• Updated all internal imports to use barrel paths (e.g., '../errors' instead of '../errors/axionveraError')
• Normalized src/estimation/index.ts to use explicit named exports
• Added src/imports.test.ts verification test

Changes

• 65 files changed, +210/-119 lines

Closes #324