fuzz_targets%: every fuzzable app, found at compile time

Elaborates to a List Cairn.Fuzz.Target, one per Machine declared in an app module (appModule: an app package's own) of the imported environment. For each machine it collects, from the claim registry and the environment:

• the App of the same state type declared in the machine's module (preferring <ns>.app);
• the keyed moves of the machine's namespace;
• the invariants stated against it, with their check when invariant generated one;
• every claim the module declares;
• <ns>.fuzz : Cairn.Fuzz.Custom S, if the app defines one.

So an app needs no fuzz code: importing it from its lean_exe fuzz root is enough (Cairn's test/fixture/lean/FuzzMain.lean). Compile time only (imports Lean); the fuzz executable is the only thing that uses it.

Whose machines. A machine whose module is in the fuzz executable's own Lake package (the package Lake builds the root in) is the package's: its target is named after its module's last component (Fixture, Claims), with the machine's name added when the module declares several (Gallery.machine). A machine of a dependency (cairn_atproto's session, imported by an app) is collected too, marked dependency, and named with its package first: cairn_atproto/Claims. lake exe fuzz runs only the package's own unless asked (--deps, docs/fuzz.md). Two targets with one name are an error here, never one shadowing the other (cairn#221).