ADR-113: Fail-closed tool data-class enforcement switch 

Context 

The composite tool gate (ADR-094) added a trust-zone axis that denies a tool whose data class exceeds the configuration's trust zone. It is governed by the tools.dataClassEnforcement extension setting so operators can watch it before turning it on: observe computes and reports the decision but still offers the tool, any other value enforces.

The switch read fail-open: it enforced only on an exact enforce and observed on everything else — a typo (enforced, ENFORCE), an empty value, a malformed tools section, or an extension configuration that threw on read all silently OBSERVED. A security gate that disables itself on a misconfiguration is exactly backwards: the operator believes the gate is on while a mistyped setting leaves it off.

Decision 

The switch is fail-closed. The axis observes ONLY on a deliberate observe (matched case- and whitespace-insensitively). Every other case enforces:

• a missing value or tools section;
• a malformed section (tools is not an array, the value is not a string);
• a typo or any unrecognised value;
• an extension configuration that throws on read.

This cannot over-permit: the four pre-existing gates always enforce, and turning the trust-zone axis on only ever removes an over-ceiling tool — so failing closed is strictly safer, never looser. An operator who genuinely wants observe-only must say so explicitly.

Consequences 

• The shipped default is unchanged in this step: ext_conf_template.txt still sets tools.dataClassEnforcement = observe, so a healthy install (fresh or upgraded) reads an explicit observe and behaves exactly as before. Only a broken, missing, or mistyped setting changes — from a silent observe to a safe enforce.
• Flipping the shipped default to enforce for new installs (while preserving observe for existing ones via an upgrade wizard) and a readiness report are a follow-up: they change behaviour for healthy installs and carry a backwards-compatibility decision, kept separate from this pure fail-closed fix.