docs: port cross-repo map from owl/REPO_MAP.md into claude-shared

[Babissimo]

Migrates the non-duplicated parts of the local owl/REPO_MAP.md into the org source of truth so there's one place for cross-repo architecture, and modernizes the diagrams.

What's here

Content ported from REPO_MAP.md

• architecture.md §6 — cross-repo connection matrix (who-calls-whom)
• architecture.md §4 — live-endpoints table (prod nodes, *.retina.fm, LAN UIs)
• architecture.md §5 — added docs and claude-shared rows
• docs/cross-cutting-changes.md (new) — "I need to change X, where do I look?" blast-radius guide, linked from the README

Diagrams → Mermaid

• §1 (system context) and §2 (signal chain): ASCII art → Mermaid
• §6: two readable Mermaid views (runtime data/API calls; build/deploy/hardware coupling), with the exhaustive matrix kept collapsed in a <details> block as the precise reference

Corrections baked in

Adopts the survey-verified framing already in architecture.md rather than the older REPO_MAP wording:

• the vendored libs (tracker/geolocator/analytics) run in-process inside the central server, not as tracker-host subprocesses
• the node→central detection forward is flagged config-gated (was disabled on the node surveyed)

Review notes

• Please confirm all four Mermaid diagrams render (GitHub renders them in the diff / file view).
• Not ported (deliberately): per-repo deep detail — that belongs in each repo's own CLAUDE.md via setup-repo, not the org doc.
• Follow-up on the owl side: REPO_MAP.md has been trimmed locally to point here for the migrated sections.

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