A frozen prior schema must not share a schema helper

Trigger: You are about to widen a schema helper such as LineChartSchema(). Before editing, check who else calls it:

rg "LineChartSchema\(\)" internal/provider/dashboards/dashboard_schema

If a v1.go..v3.go calls it, your new attribute lands in that prior version too.

Fix: Give the prior version its own frozen copy and point it there. Name it for the version it serves, keep it in the same package, and say in a comment that it must not be redirected back:

// v3.go
"line_chart": dashboardwidgets.LineChartSchemaV3(),   // frozen snapshot, not the shared helper

TestPriorSchemasStayFrozen in internal/provider/dashboards/dashboard_schema/frozen_prior_schema_test.go fingerprints V1 to V3 and fails on any change, including one made through a shared helper. Run it after touching any helper; a failure means you widened a prior schema, not that the fingerprint is stale.

Why: Prior schema versions exist only to decode state written by older provider releases. Widening one changes how that old state is interpreted. Most prior versions in this repo declare widgets inline for exactly this reason — a shared helper is the exception, and it is the one that bites. Duplication is correct here: a frozen snapshot is meant to drift from the current schema.