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/README.md

@@ -0,0 +1,56 @@
+# Bux benchmarks (E.5)
+
+Micro-kernels + optional Nexus HTTP throughput for regression and language comparison.
+
+## Quick run
+
+```bash
+# From repo root
+make bench              # Bux micro + C + Nim (+ Zig if installed)
+make bench-nexus        # wrk vs apps/nexus /api/health
+BENCH_NEXUS=1 make bench
+```
+
+## Micro suites
+
+| Suite | Kernels | Build |
+|-------|---------|-------|
+| `micro/` | Bux: `int_loop`, `fib30`, `string_concat`, `array_push` | `buxc build` |
+| `c/` | C: `fib30`, `int_loop` | `gcc -O2` |
+| `nim/` | Nim twins | `nim c -d:release --opt:speed` |
+| `zig/` | Zig twins (optional) | `zig build-exe -OReleaseFast` |
+
+Output lines:
+
+```
+BENCH <name> iters=N total_us=T us_per_op=P
+```
+
+## Nexus throughput
+
+`tools/bench_nexus.sh` / `make bench-nexus`:
+
+1. Builds `apps/nexus`
+2. Starts on `NEXUS_PORT` (default **18080**), bind `127.0.0.1`
+3. `wrk -t4 -c64 -d5s` against `/api/health`
+4. Prints `BENCH nexus_health rps=…`
+
+Env knobs:
+
+| Variable | Default | Meaning |
+|----------|---------|---------|
+| `NEXUS_PORT` | `18080` | Listen port (also used by the server binary) |
+| `NEXUS_WORKERS` | `4` | Worker threads |
+| `NEXUS_BENCH_DURATION` | `5s` | wrk `-d` |
+| `NEXUS_BENCH_C` | `64` | wrk connections |
+| `NEXUS_BENCH_T` | `4` | wrk threads |
+
+Server-side (any nexus run):
+
+- `NEXUS_PORT`, `NEXUS_WORKERS`, `NEXUS_BIND`, `NEXUS_PUBLIC`
+
+## Notes
+
+- Numbers vary by machine; use them relatively (same host, same day).
+- Nexus currently closes connections (`Connection: close`) — RPS is honest for that model, not keep-alive maxed.
+- Requires `wrk` for `bench-nexus` (`apt install wrk` on Debian/Ubuntu).