Stabilize `parse()` with `std::expected`

[K20shores]

See #268. Replace UniversalParser/ParserResult with parse(path)/parse(string) returning std::expected<Mechanism, Errors>. Dispatches to the correct version parser (v0, v1, v2), each producing the canonical Mechanism. Structural and semantic errors both reported. development::types::Mechanism becomes mechanism_configuration::Mechanism. ConfigParseStatus renamed to ErrorCode.

Acceptance criteria

• parse(<filepath>) returns std::expected<Mechanism, Errors> with all errors included
• .map() and .map_error() compose correctly
• No caller touches UniversalParser
• No v0::types or v1::types remain; all parsers target the canonical mechanism types.
• development::types becomes the canonical type without a version, so it's just a mechanism_configuration::Mehchanism, mechanism_configuration::Arrhenius, etc.

Ideas

• Semantic checks (species referenced in reactions exist in the species list, etc.) run after parsing
  • These means that two possible paths are parse file -> validate -> mechanism, or in code mechanism -> validate -> mechanism

[K20shores]

Likely will be able to address #238 with this