docs(architecture): add module map + data flow + core abstractions

Walks a new contributor through the library's src/ layout, the
parse/build data flow, the four core abstractions (PackageURL,
PurlBuilder, PurlComponent, PurlType), and where ecosystems plug
in. Diagrams show dependency direction (no cycles) and the data
path for parsing vs building a PURL.

Junior-dev level: introduces PURLs as a concept before diving in,
names every module in the src/ tree once with a one-line purpose,
and closes with a "where to look when I want to do X" section.

Author: jdalton