feat: unify name demangling with single Lean implementation

This PR replaces three independent name demangling implementations (Lean,
C++, Python) with a single source of truth in Lean. The new
`Lean.Compiler.NameDemangling` module handles the full pipeline: prefix
parsing, postprocessing (suffix flags, private names, hygienic stripping,
specialization contexts), backtrace line parsing, and C exports. The C++
runtime backtrace handler now calls `@[export]`ed Lean functions instead
of its own reimplementation. The Python profiler demangler is replaced
with a thin subprocess wrapper around a Lean CLI tool.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kim-em

script/profiler/lean_demangle.py

@@ -0,0 +1,32 @@
+/-
+Copyright (c) 2025 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Kim Morrison
+-/
+module
+
+import Lean.Compiler.NameDemangling
+
+/-!
+Lean name demangler CLI tool. Reads mangled symbol names from stdin (one per
+line) and writes demangled names to stdout. Non-Lean symbols pass through
+unchanged. Like `c++filt` but for Lean names.
+
+Usage:
+    echo "l_Lean_Meta_foo" | lean --run lean_demangle_cli.lean
+    cat symbols.txt | lean --run lean_demangle_cli.lean
+-/
+
+open Lean.Name.Demangle
+
+def main : IO Unit := do
+  let stdin ← IO.getStdin
+  let stdout ← IO.getStdout
+  repeat do
+    let line ← stdin.getLine
+    if line.isEmpty then break
+    let sym := line.trimRight
+    match demangleSymbol sym with
+    | some s => stdout.putStrLn s
+    | none => stdout.putStrLn sym
+    stdout.flush