[mbrunnen]

Fixes #29. Follow-up to #10.

The context stage now sets Diagnostic::code for the findings that name a
rule, so check --format json reports their rule. Six owning-type rules
and the name clash get their rule name, and four semantic findings name the
rule they report. No finding is added or removed, and no severity changes.

How to test

$ cargo test -p sysmlv2-cli --test integration check_format_json_names_the_rule_of_every_stage
$ cargo test -p sysmlv2-parser --test integration opensysml::

Look closely at

• sysmlv2-syntax/src/check.rs gets a rule_error helper that writes
  "rule: message" and sets the code, the style the context stage already
  used. The semantic stage keeps its "message [rule]" style.
• The new names are in IMPLEMENTED_NORMATIVE_VALIDATION_RULES and
  IMPLEMENTED_NORMATIVE_SEMANTIC_RULES, and the XMI audit accepts them.
• The opensysml baselines move only in message text (24 and 22 lines), one
  for one. No finding is added or lost.
• A verify member in the wrong body had a context and a semantic finding
  before; both now name validateRequirementVerificationMembershipOwningType.
• The CHANGELOG.md entry is under a new Unreleased heading. Move or drop
  it as you prefer.