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

bootstrap/lir_c_backend.nim

@@ -10,11 +10,17 @@ type
     output*: string
     indent*: int
     tempTypes*: Table[string, string]  ## Track C types of temp variables
+    emitDebugLines*: bool              ## Emit #line → .bux for DWARF (E.4)
+    lastDebugLine*: int
+    lastDebugFile*: string
 
-proc initLirCBackend*(): LirCBackend =
+proc initLirCBackend*(emitDebugLines: bool = true): LirCBackend =
   result = LirCBackend(
     indent: 0,
     tempTypes: initTable[string, string](),
+    emitDebugLines: emitDebugLines,
+    lastDebugLine: 0,
+    lastDebugFile: "",
   )
 
 proc emitIndent(be: var LirCBackend) =
@@ -26,6 +32,22 @@ proc emitLine(be: var LirCBackend, s: string) =
   be.output.add(s)
   be.output.add("\n")
 
+proc emitDebugLine(be: var LirCBackend, instr: LirInstr) =
+  ## Map generated C back to Bux source for gdb/DWARF via #line.
+  if not be.emitDebugLines: return
+  if instr.locLine <= 0: return
+  if instr.locLine == be.lastDebugLine and instr.locFile == be.lastDebugFile:
+    return
+  be.lastDebugLine = instr.locLine
+  be.lastDebugFile = instr.locFile
+  var path = instr.locFile
+  if path.len == 0:
+    path = "<bux>"
+  # Escape for C string literal
+  path = path.replace("\\", "\\\\").replace("\"", "\\\"")
+  # #line must start at column 0
+  be.output.add(&"#line {instr.locLine} \"{path}\"\n")
+
 proc valToC(be: var LirCBackend, v: LirValue): string =
   ## Convert a LirValue to its C representation.
   case v.kind
@@ -50,6 +72,7 @@ proc cParamDecl(cType, name: string): string =
 # ── Per-instruction emission ──
 
 proc emitInstr(be: var LirCBackend, instr: LirInstr) =
+  be.emitDebugLine(instr)
   template v(x: LirValue): string = valToC(be, x)
   case instr.kind
 
@@ -246,6 +269,16 @@ proc emitFunc(be: var LirCBackend, f: LirFunc, funcRetTypes: Table[string, strin
   if f.params.len == 0:
     paramsStr = "void"
 
+  # Point the function entry at the first Bux location so gdb `list Main` works
+  # (otherwise leftover #line from the previous function pollutes the prologue).
+  if be.emitDebugLines:
+    for instr in f.instrs:
+      if instr.locLine > 0:
+        be.lastDebugLine = 0
+        be.lastDebugFile = ""
+        be.emitDebugLine(instr)
+        break
+
   be.emitLine(&"{f.retType} {f.name}({paramsStr}) {{")
   be.indent += 1