Make the model chapters agree with themselves

Parent

Wording fixes for rules the model has already decided, so that a reader of any one chapter meets the rule in force. No model change.

Hardening: chapter 20's "one platform posture plus declared exceptions" and its authority row "from hardening and its declared exceptions" lose the exceptions. CONTEXT.md's Hardening Class says there is no per-Process exception (0016).
"a Context", "the pinned context" and "a context" in chapters 10, 16 and 60 become Platform Intent, the term CONTEXT.md uses since the Cluster Context was retired.
Chapter 10's duplicated sentence fragment about writing a derived value as a literal is removed.
Chapter 60's onboarding step says an Application carries an optional observability block, not alertClass. Its platform-facts paragraph lists the monitor cadence, not an observability policy.
CONTEXT.md's Alert Class says how urgently a signal should wake someone; routing belongs to the monitoring stack (chapter 10: "urgency, never routing").
Chapter 16's derivation-map mermaid no longer labels Platform Intent as layer 2, and drops the Flux health timeout class, Flux Kustomization, reachability entry and edge-catalog nodes that its drawn SVGs already dropped.
Chapter 10's file-layout rule and the worked examples agree on the project file name and on the per-Process env file path. Four of the five example env files do not follow the chapter today.