Split API route modules

[yvette-carlisle]

Summary

• split top-level API router construction into a route_builder module
• split graph, knowledge, notes, docs, and trace route handlers into focused Rust submodules
• update the trace replay route-source scan to include the route module tree after modularization

Local validation

• cargo make check

Diff size

• 23 files changed, 1742 insertions(+), 1558 deletions(-)