feat: HTTP registry, app/bench harnesses, and DWARF #line maps

Ship QUALITY_PLAN sessions 31–33: fetchable package index URLs, showcase
app and micro/nexus benchmarks, and debugger-friendly C codegen.

- Registry: BUX_REGISTRY accepts http(s) URLs (curl/wget → ~/.bux/cache)
- E.2: make test-apps smoke for nexus/boko/simpledb/jwt-pitbul
- E.5: benches/micro + C/Nim/Zig twins; make bench-nexus (wrk)
- E.4: HIR locs → #line .bux; default -O0 -g; --release -O2; make test-dwarf

benches/micro/bux.toml

@@ -0,0 +1,8 @@
+[Package]
+Name    = "micro"
+Version = "0.1.0"
+Type    = "bin"
+Description = "Micro-benchmarks for Bux (E.5)"
+
+[Build]
+Output = "Bin"