Modularize core block and docs surfaces

[yvette-carlisle]

Summary

• split core block type and validation surfaces into explicit Rust submodules
• split source capture helpers, docs source-ref validation, and document storage query modules
• split knowledge watch candidate construction/proposal/run helpers into focused modules

Validation

• RUSTC=/run/current-system/sw/bin/rustc cargo make fmt-rust && RUSTC=/run/current-system/sw/bin/rustc cargo make check after each segment

Size

• 34 files changed, 1668 insertions, 1526 deletions