feat: add Lean name demangling to runtime backtraces

This PR adds human-friendly demangling of Lean symbol names in runtime
backtraces. When a Lean program panics, stack traces now show readable
names instead of mangled C identifiers.

Before: `l_Lean_Meta_Grind_unfoldReducible___redArg___lam__0___boxed`
After:  `Lean.Meta.Grind.unfoldReducible [boxed, λ, arity↓]`

The demangler is a C++ port of `Name.demangleAux` from NameMangling.lean
with human-friendly postprocessing:
- suffix folding: `_redArg` → `[arity↓]`, `_boxed` → `[boxed]`, `_lam_N` → `[λ]`
- private name cleanup: `_private.Module.0.Name.foo` → `Name.foo [private]`
- specialization context: `_at_`/`_spec` → `spec at ...`
- hygienic suffix stripping
- `lean_apply_N` → `<apply/N>`

Set `LEAN_BACKTRACE_RAW=1` to disable demangling and get raw symbol names.

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