Add `{.raises.}` annotations to `tests` files where needed to enable
`{.push raises: [].}`. Avoids interfering with periodic changes such as
spec version bumps, and avoids special casing folders when editing.
The effort to maintain `{.raises.}` is trivial after the initial round.
Distinguish between those code locations that need to be updated on each
light client data format change, and those others that should generally
be fine, as long as a valid light client object is processed.
The former are tagged with static assert for `LightClientDataFork.high`.
The latter are changed to `lcDataFork > LightClientDataFork.None` to
indicate that they depend only on presence of any valid object.
Also bundled a few minor cleanups and fixes.
Also add `Forky` type for `LightClientStore` and minor fixes / cleanups.