Allow formatting semantically invalid source

[konojunya]

Background

The Task 8 formatter contract requires syntactically valid source with semantic errors to remain formattable. The initial canonical formatter specification incorrectly described every compiler-stage error as a formatting failure, which conflicts with diagnostic-preserving editor and CLI workflows.

Implementation

• limit formatter rejection to lexical and syntax errors
• state explicitly that semantic and complexity diagnostics do not prevent formatting
• require normalized IR equality for valid documents
• require the portable diagnostic-code set to remain unchanged for syntactically valid documents with semantic or complexity errors

Testing

• python3 scripts/validate-compiler-diagnostics.py
• python3 scripts/validate-formatter-fixtures.py
• JSON Schema metaschema, normalized IR fixture, and diagnostic fixture validation
• git diff --check

Review focus / concerns

This is a contract correction only; canonical bytes and existing fixtures do not change. Successful formatting still does not claim semantic validity.