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

apps/nexus/README.md

@@ -32,9 +32,12 @@ cd apps/nexus
 
 # Run
 ./nexus
+
+# Optional env (also used by `make bench-nexus`)
+# NEXUS_PORT=18080 NEXUS_BIND=127.0.0.1 NEXUS_WORKERS=4 ./build/nexus
 ```
 
-Server starts on `http://0.0.0.0:8080`:
+Server starts on `http://0.0.0.0:8080` (override with `NEXUS_PORT` / `NEXUS_BIND`):
 
 ```
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