Variable ISSUE_TYPES`Const`

ISSUE_TYPES: readonly [
    "type_collision",
    "interpolation_error",
    "extraction_error",
    "embedding_error",
    "upsert_error",
    "match_failed",
    "rule_missing",
    "no_content",
    "source_unavailable",
    "unsupported_content_type",
] = ...

Canonical issue codes, in one list (spec Decision 16.4).

type_collision and interpolation_error predate 0.20.0; the rest cover processing failures (#255) and source documents.