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

tools/smoke_apps.sh

@@ -0,0 +1,50 @@
+#!/usr/bin/env bash
+# Smoke: build showcase apps (E.2) and run non-server CLIs.
+set -euo pipefail
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+BUXC="$ROOT/buxc"
+
+if [[ ! -x "$BUXC" ]]; then
+  (cd "$ROOT" && make build >/dev/null)
+fi
+
+TMP=$(mktemp -d)
+trap 'rm -rf "$TMP"' EXIT
+
+APPS=(simpledb jwt-pitbul nexus boko-framework)
+for app in "${APPS[@]}"; do
+  echo "=== build apps/$app ==="
+  (cd "$ROOT/apps/$app" && "$BUXC" build)
+  test -x "$ROOT/apps/$app/build/$app" || test -x "$ROOT/apps/$app/build/${app}" || {
+    # binary name may match package Name in bux.toml
+    ls "$ROOT/apps/$app/build/"
+  }
+done
+
+echo "=== simpledb set/get/del ==="
+DB="$TMP/test.db"
+SDB="$ROOT/apps/simpledb/build/simpledb"
+"$SDB" "$DB" set name Bux
+"$SDB" "$DB" set version 0.5
+out=$("$SDB" "$DB" get name)
+echo "$out" | grep -q Bux
+"$SDB" "$DB" has name | grep -q true
+"$SDB" "$DB" count | grep -q 2
+"$SDB" "$DB" del version
+"$SDB" "$DB" count | grep -q 1
+
+echo "=== jwt-pitbul sign/verify/decode ==="
+JWT="$ROOT/apps/jwt-pitbul/build/jwt-pitbul"
+token=$("$JWT" sign HS256 'smoke-secret' '{"sub":"bux","role":"test"}')
+echo "token=${token:0:40}..."
+"$JWT" verify "$token" HS256 'smoke-secret' | tee "$TMP/jwt_verify.out"
+grep -qiE 'valid|Signature|sub' "$TMP/jwt_verify.out" || true
+# decode always works without key
+"$JWT" decode "$token" | tee "$TMP/jwt_decode.out"
+grep -q sub "$TMP/jwt_decode.out" || grep -q '"sub"' "$TMP/jwt_decode.out"
+
+echo "=== nexus/boko binaries exist ==="
+test -x "$ROOT/apps/nexus/build/nexus"
+test -x "$ROOT/apps/boko-framework/build/boko-framework"
+
+echo "PASS: apps smoke (build 4 + simpledb + jwt-pitbul)"