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

config/registry.toml

@@ -1,8 +1,10 @@
-# Bux package registry index (E.1)
+# Bux package registry index (E.1 + HTTP URL)
 # Used by `bux add <name>` and `bux install` when no --path/--git is given.
 #
 # Override with: export BUX_REGISTRY=/path/to/registry.toml
+#            or: export BUX_REGISTRY=https://example.com/registry.toml
 # Or copy to:    ~/.bux/registry.toml
+# Force HTTP re-fetch: BUX_REGISTRY_REFRESH=1
 #
 # source forms:
 #   file:relative/or/absolute   — local package (relative to this file)