config: treat a top-level "//" key as a comment, not an unknown field (#505)

Strict validation of top-level config fields rejects the conventional "//" comment key that plain- JSON configs use to carry notes (JSON proper has no comments). A config carrying such a key failed validation with "unknown top-level field", breaking deployments whose stored config used the convention. Validation now skips "//" when checking for unknown top-level fields, with a test.

Stacked on #468 — merge that first.

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