Scanner area doc: record #219's rulings and measurement mode's local-only rule #231

From #219 (rulings 1–3) and #218 (ruling 6), maintainer, 2026-11-09. Mostly documentation in docs/architecture/scanner.md, which marks each of these "no spec source" today.

1. The scanner's CSP extras are policy (Scanner: three behaviours no spec describes (CSP's extra directives, the OCR worker's lifetime, a name candidate's printing) #219-1): object-src 'none', base-uri 'self', form-action 'self', frame-ancestors 'self', font-src 'self', and img-src's data: and blob: (app/controllers/concerns/scanner_page.rb). Record them as decided by Scanner: three behaviours no spec describes (CSP's extra directives, the OCR worker's lifetime, a name candidate's printing) #219, alongside spec 007 AC-2.4 and ADR 0001. Check whether any spec already pins them; if none does, add an example that checks the scanner page's header carries all of them (test-first: it's a new guard of existing behaviour, so prove it fails when one directive is removed).
2. A fresh OCR worker per page load is the rule (Scanner: three behaviours no spec describes (CSP's extra directives, the OCR worker's lifetime, a name candidate's printing) #219-2). The scanner's turbo-visit-control: reload exists so the camera is released (spec 007 FR-2). Correct the comment in app/javascript/scanner/recognition.js (it says "kept across Turbo visits"), and record the rule.
3. The read-set preference is the rule (Scanner: three behaviours no spec describes (CSP's extra directives, the OCR worker's lifetime, a name candidate's printing) #219-3): a name candidate's printing is the one in the set the collector line read, else its newest English printing (MTG::Reading#name_printings, pinned by spec/models/mtg/reading_spec.rb). Record it.
4. Measurement mode answers local requests only (Accounts and sessions: six behaviours no spec describes #218-6), on top of the mode being on, because its routes skip sign-in (MeasurementMode#require_local_request). Record it as a rule.

Each statement names its issue as source; #218 and #219 go in the doc's Sources. No spec edits (shipped specs stay frozen, spec 027 D4).

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