feat(selfhost): field-move skip Drop, #line maps, Array field mono

Bring selfhost C backend closer to bootstrap ownership and debug quality.

- Mark locals moved into struct fields / returns (no double Drop)
- Mangle Array/Set/Map/Channel field types so user structs fully emit
- Emit #line N from HIR line; BUX_DEBUG_FILE / BUX_NO_LINE controls

docs/BuildAndTest.md

@@ -148,12 +148,13 @@ make bench-nexus     # wrk throughput vs apps/nexus /api/health
 ```bash
 ./buxc build              # -O0 -g, #line → .bux (gdb-friendly; bootstrap)
 ./buxc build --release    # -O2 -DNDEBUG, no #line / -g
-# Selfhost (buxc2) same --release / default -O0 -g; #line maps bootstrap-only for now
+# Selfhost (buxc2): same --release / default -O0 -g; #line via HIR line numbers
+export BUX_DEBUG_FILE=src/Main.bux   # path embedded in #line (selfhost)
+export BUX_NO_LINE=1                 # disable selfhost #line maps
 export BUX_CFLAGS="-fno-omit-frame-pointer"   # optional extra cc flags
 gdb --args ./build/myapp
 #   (gdb) break Main
-#   (gdb) run
-#   (gdb) list            # shows Bux source via #line (bootstrap)
+#   (gdb) list            # Bux source via #line
 ```