Implement AST multi-error recovery

[LesterEvSe]

Analysis now reports every independent error in one pass instead of stopping at
the first.

• Poison, not fake values; Error variants in the parse tree, AST, and type
  system (TypeInner::Error); recovery emits these instead of fabricated units,
  aliases, or sizes.
• No cascades; poisoned types are absorbed during analysis (is_error guards
  and ResolvedType::compatible), so one broken construct does not spawn secondary
  errors.
• Collect, do not abort; Scope carries a diagnostics sink; containers record
  an error and keep analyzing siblings, and Program::analyze returns Option.
• Parser split; parse_from_str (fragments, first error) vs
  parse_from_content (files, collect all), over one shared core.