[ewiger]

Closes issues 0052 and 0053: every defect the line-by-line review of the complete type system branch confirmed, one commit per unit of the work plan in doc/issues/0053-review-of-unreviewed-code.md, each with a regression test.

• Parser: no panic on an argument list cut off at end of input; fork branches are full pipelines and need a separator; destructuring across lines and with a trailing comma; import and impl usable as names; type X = before a line break; empty-bracket message.
• Checker: operators resolve through their own trait only; Self is the implementing type; a generic record re-checks its fields once bound, and its parameters take their variance from the fields; equality reaches a user Eq through Option; none covers Option<Never>.
• Equality: a container is equal by its type's rule and its elements by their own == (stated in the type-system card); collection steps compare as == does.
• Collections: take orders a Set by Ord; collect, emit, flat_map take finite iterating functions.
• Sources and boundary: SQLite reads follow column types (Bool, Float from integers, a NUMERIC Decimal refused as lossy), views and WITHOUT ROWID tables read, non-UTF-8 blobs refused, an unreadable hql.toml is an error, a dotted name is not a path, column names match without regard to case, swapped external names and path fields decode, a Decimal written to SQLite keeps its digits.
• Performance: distinct, merge, count_by, histogram and map construction are no longer quadratic.
• decimal.rs takes gcd from num-integer (already in the lock file).

585 tests pass; clippy and fmt are clean.