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/nim/fib.nim

@@ -0,0 +1,16 @@
+## Nim twin of benches/c/fib.c — recursive fib(30)
+import std/[times, strformat]
+
+proc fib(n: int): int =
+  if n <= 1: return n
+  fib(n - 1) + fib(n - 2)
+
+proc nowUs(): int64 =
+  int64(epochTime() * 1_000_000.0)
+
+discard fib(20)
+let t0 = nowUs()
+let r = fib(30)
+let t1 = nowUs()
+if r < 0: quit(1)
+echo &"BENCH fib30 iters=1 total_us={t1 - t0} us_per_op={t1 - t0}"