fix: apply live data bindings in nemo screenshot (#82)

[geoffjay]

Summary

Fixes #82 — nemo screenshot rendered the initial layout but bound values (<binding>/bind-*) stayed at their placeholders in the captured PNG, even though they populate correctly in the running app.

Root cause is startup ordering, not a screenshot-specific rendering bug:

• A data source's start() broadcasts its initial full value immediately.
• A tokio broadcast channel drops messages sent while no receiver is attached.
• The runtime subscribed its update loops after data_engine.start_all(), so each source's first value was lost and the repository was never seeded.
• Interactive runs recover via a later event (e.g. a file-watcher change); a one-shot capture never does.

Changes

• runtime.rs (root-cause fix): subscribe the update loops (start_data_update_loop) before data_engine.start_all(), so receivers are attached before sources broadcast. Benefits every one-shot / early-read path, not just screenshots.
• commands/screenshot.rs (capture fix): pump the data→binding loop across the --settle-ms window in 50ms slices, then do a final refresh + one executor-yield before render_to_image() (which reads the last-drawn frame; refresh() only marks the window dirty).
• KB: added a "Startup ordering is load-bearing" note to concepts/data-flow.md and a log.md entry.

This branch also carries an earlier doc-only commit adding the screenshot-as-a-plugin KB plan.

Verification

• examples/data-binding screenshot now shows live timer/HTTP values instead of placeholders.
• 233 nemo + 18 macro tests pass; no regressions from the runtime reorder.
• cargo build -p nemo --features screenshot clean; pre-commit clippy/fmt hooks green.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.59%. Comparing base (d906c33) to head (f8d4a58).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #86   +/-   ##
=======================================
  Coverage   56.59%   56.59%           
=======================================
  Files          78       78           
  Lines        6866     6866           
=======================================
  Hits         3886     3886           
  Misses       2980     2980           

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