Add #cpp_heap_allocated directive, deprecate #cpp_value

[camio]

Non-AI synopsis

I added #cpp_heap_allocated to replace #cpp_value to address #143.

I left #cpp_value in for backward compatibility. If you use it, you get a warning.

There wasn't an effective warning mechanism in place, so I added one as well. There was some complexity because I wanted to ensure "cargo:warning=" is prefixed, so you get warnings when you use zngur through cargo. I'm not happy with the add_errors, add_error_str, and add_warning_str set of functions. It doesn't feel like a very good API, but fixing it would require deeper refactors, I fear.

What's below is AI commentary...

Summary

• Adds #cpp_heap_allocated "CppType"; as a replacement for
  #cpp_value "field" "CppType";, dropping the field parameter that
  is parsed but never used by codegen (closes #cpp_value's first argument (field) is parsed but never used #143).
• #cpp_value remains supported for backward compatibility: it now
  forwards to the same internal logic as #cpp_heap_allocated and
  emits a deprecation warning (rendered as an ariadne diagnostic
  pointing at the directive's location, including across imported
  .zng files).
• Warnings are delivered via Zngur::with_warning_sink() (defaulting
  to Cargo's cargo:warning= convention when used from a build
  script) and via plain eprintln! in zngur-cli.
• Internal representation renamed from CppValue/cpp_value to
  CppHeapAllocated/cpp_heap_allocated throughout zngur-def,
  zngur-parser, and zngur-generator, following the same
  parse-time-struct-plus-generator-produced-struct convention already
  used for ZngurField/ZngurFieldData.
• Documentation and the two example .zng files updated to use the
  new directive.

Test plan

• cargo test across zngur-def, zngur-parser, zngur-generator,
  zngur, zngur-cli (37 parser tests, all passing)
• examples/tutorial_cpp (build.rs path) builds and runs correctly
  with the new directive, and with #cpp_value temporarily
  reintroduced, correctly shows the cargo:warning=-rendered
  deprecation diagnostic
• examples/multiple_modules (CLI path via its Makefile) builds
  and runs correctly, matching expected_output.txt
• cargo fmt --check clean on all touched crates

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