Add more Decoder helpers

```hs
toDecoder :: DecodeM a -> Decoder a
toDecoder m = makeDecoder (const m)

applyDecoder :: Decoder a -> Value -> Decoder a
applyDecoder decoder = toDecoder . runDecoder decoder

-- Type mismatch: Expected String, got:
typeMismatch :: Text -> Decoder a
typeMismatch' :: Text -> Value -> DecodeM a

-- Invalid value: , got:
invalidValue :: Text -> Decoder a
invalidValue' :: Text -> Value -> DecodeM a
```
hide `DecodeM` functions; recommend `invalidValue` or `typeMismatch` over `fail`

Locate the Decoder, DecodeM, and Value definitions, then inspect the existing fail-based decoding paths and public exports. Confirm how the proposed helpers should construct type-mismatch and invalid-value errors, hide DecodeM functions from the public API, and verify that callers can use invalidValue or typeMismatch instead of fail.

Written by the indexing model from the issue text.