Prototype-key defect (#551 class) remains in three user-keyed maps outside placement maps

What

Three more plain-object maps are built by assigning a user-authored string key directly (out[key] = …), which is the same defect #551 fixed for Dashboard placement maps. A key of __proto__ invokes the inherited Object.prototype setter and creates no own property, so the entry silently vanishes; a bare read of the same key resolves up the prototype chain to Object.prototype itself, which a merge-then-rewrite can then mutate realm-wide.

Sites

• src/dashboard/model/dashboard-variable-store.ts:66,76,93,96 — out[variableId], out[dashboardId]
• src/workspace/workspace-dashboards.ts:133 — configs[name] = config
• src/dashboard/application/dashboard-viewer-session.ts:1495-1496 — proposedValues[variable.def.parameter]

All three are keyed by strings the user authors: variable IDs, dashboard IDs, and variable parameter names (the {name:Type} placeholder text in panel SQL). Nothing constrains those away from __proto__ or constructor.

Why this is cheap to fix

#551 already landed the two shared primitives in src/core/saved-query.ts and exported them:

• defineJsonField(target, key, value) — Object.defineProperty-based write
• readJsonField(target, key) — own-property-only read, for any map that gets merged, mutated, or where "no entry" differs from "an empty entry"

So this is a mechanical sweep plus round-trip tests with __proto__/constructor keys that assert the entry survives (Object.hasOwn + exact value), not merely that nothing throws.

Why deferred

Out of scope for #551, which was explicitly about placement maps keyed by tile ID — one sweep across every site it named, rather than quietly widening that PR's diff. Found while doing that sweep (PR #558).

Note on severity

#551's body originally claimed "no prototype pollution escapes". That is not true of the read path: setStylePlacement was reproduced polluting Object.prototype.grid for the whole realm. Whether any of the three sites above is reachable that way has not been checked — each needs the read side examined, not just the write.

Start with the three listed sites: src/dashboard/model/dashboard-variable-store.ts, src/workspace/workspace-dashboards.ts, and src/dashboard/application/dashboard-viewer-session.ts. Read the defineJsonField and readJsonField primitives in src/core/saved-query.ts, then add round-trip tests for proto and constructor keys. Done means each entry survives with Object.hasOwn and its exact value, including any relevant read paths.