fix(compiler): resolve, name, and load namespaces by source extension (#116)

[skydread1]

Closes #116

• source-extensions becomes [".cljr" ".cljc" ".clj"] in both copies, matching ClojureCLR.
• compile-file derives the assembly name from the source extension, so foo.cljc emits foo.cljc.dll.
• -load takes its assembly candidate from the resolved source's extension instead of searching independently, so a stale foo.clj.dll cannot win an mtime comparison against foo.cljc.
• Pairing the assembly with its source is a deliberate divergence from ClojureCLR. No library source changes, so libraries still load on stock cljr.
• IsCljAssembly matches all three suffixes, and the coexistence import hooks use it instead of their own check.
• The smoke suite's interop namespace becomes .cljr, giving the IL2CPP build a mixed assembly set.
• A test pins the two copies of source-extensions in agreement, since build.clj compiles magic.api against the previous clojure.core.