v0.4.2

Added

• Structured diagnostic codes (spec §16.4, appendix C). Every compiler and runtime diagnostic
  now carries a first-class code and renders rustc-style as error[E0011]: / warning[W0003]: /
  error[R0005]: instead of burying the code in the message text. SyntaxError/TypeError gained
  code/help fields and RuntimeError gained code() plus a Coded variant, so the
  E####/R#### tables in appendix C are now enforced by the implementation.
• Previously unimplemented appendix C validations. E0041 duplicate_definition (same-scope
  fn/class/const, duplicate parameters, duplicate fields, duplicate methods), E0051 missing_type_ann (@builtin/@c_api::extern signatures must be typed), E0052 unknown_type
  (with did you mean, covering let/const/parameter/return annotations), E0063 self_not_first, E0081 op_overload_bad_arity, and runtime R0002 underflow (integer collapse
  below the target minimum, distinct from R0001 overflow).
• Actionable suggestions. did you mean ...? now covers undefined names (E0040), unknown
  types (E0052), unknown/missing fields (E0060/E0061), unknown config keys (E0022),
  @builtin optimization tiers (E0057), call-site arity, and method-name typos, alongside
  rule-based hints (for example the explicit-collapse hint for Expr/numeric mismatches).
• --json machine-readable diagnostics. All diagnostics can be emitted as rustc-style NDJSON
  on stderr (severity/code/message/spans/notes/help), keeping the program's stdout clean
  for editors and CI.
• --color auto|always|never and --quiet/-q global switches.
• Project scaffolding: prima new <name> and prima init write the spec §20 layout
  (prima.toml / config.toml / README.md / .gitignore / src/main.pra).
• Project-root discovery: prima run / check / fmt / doc accept an omitted file and use
  the nearest ancestor prima.toml (or src/main.pra); prima test defaults to src/ inside a
  project.
• prima completions <shell> emits a shell completion script via clap_complete.
• prima test --json emits one NDJSON event per test ({"type":"test","file","status","message"})
  plus a final {"type":"summary","passed","failed","skipped"} on stderr; --quiet suppresses
  passing/skipped lines while keeping failures and the summary.

Changed

• REPL errors now print the structured error[CODE]: header, and location-less runtime errors keep
  their R#### code.