Reconcile numeric expression parsing with measured GitHub behavior

[kjanat]

Follow-up from #47. Its original lexer inventory predates subsequent changes and must be rechecked.

The register identified rejected forms accepted by GitHub, including hexadecimal literals with leading zeros and integers above int32, and pointed to 32-bit parsing in expr_parser.go. #29 addressed part of the original inventory.

• Compare the current lexer/parser and tests with the preserved expression reference and probes.
• Identify remaining mismatches, separating literal parsing from string-to-number coercion and intentional restrictions from defects.
• Fix confirmed parser defects with focused regression tests; document any deliberately retained restrictions.