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/zig/int_loop.zig

@@ -0,0 +1,22 @@
+// Zig twin of benches/c/int_loop.c — 1e6 integer arithmetic loop
+// Build: zig build-exe -OReleaseFast -femit-bin=build/int_loop int_loop.zig
+const std = @import("std");
+
+fn nowUs() i64 {
+    return @divTrunc(@as(i64, @intCast(std.time.nanoTimestamp())), 1000);
+}
+
+pub fn main() !void {
+    const iters: i32 = 1_000_000;
+    var acc: i32 = 0;
+    const t0 = nowUs();
+    var i: i32 = 0;
+    while (i < iters) : (i += 1) {
+        acc = acc + i * 3 - 1;
+    }
+    const t1 = nowUs();
+    if (acc == 0 and iters > 0) return error.Unreachable;
+    const total = t1 - t0;
+    const out = std.io.getStdOut().writer();
+    try out.print("BENCH int_loop iters={d} total_us={d} us_per_op={d}\n", .{ iters, total, @divTrunc(total, iters) });
+}