Let debug logs follow initialized log level

[funwithcthulhu]

Summary

• Let sub-logs without an explicit level inherit the global Logs level instead of freezing the current Dream default at source creation time.
• Preserve explicit per-source levels set through Dream.sub_log ~level and Dream.set_log_level.
• Add a unit test for the Dream.debug case from the issue: the default logger is created before Dream.initialize_log ~level:Debug ()`, then debug output should be enabled.

Closes #388.

Validation

• git diff --cached --check
• opam exec -- dune runtest test/unit (blocked locally: this switch is missing alcotest)
• opam exec -- ocamlformat --check ... (not run: project requires ocamlformat 0.25.1, local switch has 0.29.0)
• opam exec -- dune build src/server/log.ml (timed out locally without a useful compiler diagnostic; stopped the leftover build process and cleared its stale _build/.lock)