Correct system diagrams

[Jason-Doyle]

Summary

• correct README read flow so HEAD retrieval, immutable-object retrieval, and the 16 MiB decoded-object boundary are separate stages
• limit the README mutation flow to document create, replace, delete, and restore operations; document administrative maintenance separately
• correct the trust diagram so the deployment master key remains inside the platform boundary and only derived authorised scope keys reach the browser
• model read-bundle fallback after the bundle request rather than as a pre-request decision
• show cached HEAD handling, decoded-object limits, bounded write retries, and explicit retry exhaustion
• regenerate the four affected content-addressed SVGs; the other twelve diagrams remain unchanged

Validation

• rendered all four README Mermaid blocks with Mermaid CLI 12
• npm run site:diagrams
• npm run verify:docs
• npm run site:check
• npm run site:build
• npm run site:test
• npm run site:deploy:check