feat(monopam): add mono=true nested monorepo support and merge mono.lock into sources.toml

Add mono and ref_ fields to sources_registry entries, enabling nested
monorepos (mono=true) with recursive push/pull and replacing mono.lock
with ref pinning in sources.toml. Rename url→source and
default_url_base→origin with full backward compatibility.

Also fix pre-existing syntax errors and formatting in test files.

fuzz/dune

···
  (modules fuzz_hap)
  (libraries hap crowbar))
 
+(executable
+ (name gen_corpus)
+ (modules gen_corpus)
+ (libraries unix))
+
 (rule
  (alias runtest)
  (deps fuzz_hap.exe)