v0.1.8: Actionable error contracts — upstream framing, wire recovery hints, canvas DX

Actionable error contracts — upstream framing, wire recovery hints, canvas DX

Changed:

• Unknown-variable rejections across the six weather/environment tools lead with actionable guidance, name the offending value(s), and demote the raw upstream Swift type-init string to a trailing parenthetical (frameInvalidVariableMessage) (#7)
• openmeteo_dataframe_describe / openmeteo_dataframe_query document openmeteo_get_ensemble alongside openmeteo_get_historical as canvas producers (#8)

Fixed:

• Every ctx.fail site attaches the declared recovery hint to the wire via ctx.recoveryFor — data.recovery.hint now ships (#11)
• canvas_id returned only when truncated: true — no pointer at an empty canvas on the non-spilled path (#8)
• Dataframe tools rethrow unknown-canvas under their own canvas_not_found contract instead of the framework's misleading omit-canvas_id hint (#8)

100 tests pass; bun run devcheck clean.