v0.8.9

• Subshell code generation: Fixed critical bug where normal command
  substitution subshells ($(...)) and cat‑replacement subshells
  ($(< file)) incorrectly shared buffer placement logic.
  • Cat‑replacement subshells now enforce perfect forwarding: pre‑code
    and post‑code are placed strictly outside the substitution, and
    local object destruction is deferred until after buffers are flushed.
  • Normal subshells now correctly scope pre‑code, post‑code, and
    destructor calls inside the substitution, ensuring proper lifetime
    management and output capture.
  • Added test cases for cat‑replacement and corrected expectations in
    the local‑scope test.
• AST pretty‑printing: SubshellSubstitution nodes now display whether
  they represent a cat‑replacement subshell, aiding debugging.
• Parser error recovery: The parser now attempts to recover from syntax
  errors and continue building the AST, allowing multiple parser errors to
  be reported in a single compilation pass rather than aborting on the
  first failure.
• bpp‑lsp improvements:
  • Removed unused --port and --socket options; the language server
    now communicates exclusively over stdio.
  • Eliminated unnecessary signal handlers; the server simply exits on
    signals as no extra cleanup is required.
  • Added a note to the help text describing LSP usage.
• Code hygiene and refactoring:
  • Replaced system() with fork() + execvp() for runs-on-exit
  • Changed in_supershell boolean flag to a stack<std::monostate>
    to correctly track nested supershell contexts.
  • Unified multiple NullOStream implementations into a single common
    definition.
  • Migrated from realpath() to std::filesystem::canonical().
  • Refined argument parsing: Bash version string parsing moved into a
    constexpr constructor of BashVersion.