fix(spec): preserve exact schema and temporal value semantics

[aarushigupta132]

Which issue does this PR close?

Prerequisite for the focused UpdateSchema series split from #3069.

What changes are included in this PR?

• Parses and formats the full Iceberg i32 date and i64 microsecond timestamp domains instead of being limited by chrono calendar ranges.
• Preserves exact microsecond fractions and handles extended proleptic-Gregorian years in JSON and display paths.
• Serializes pre-epoch nanosecond timestamps with the correct Euclidean second/nanosecond split.
• Makes case-insensitive schema lookup reject names that differ only by case instead of selecting one nondeterministically.
• Makes schema equivalence independent of identifier-set iteration order and consistent with Java floating-point equality for defaults.

This is independently mergeable and contains no transaction or schema-evolution API.

Are these changes tested?

Yes. Coverage includes temporal range boundaries, pre-epoch microsecond and nanosecond values, extended years, offsets and fractional rendering, ambiguous schema names, identifier-set ordering, NaNs and signed zero. All 1,591 iceberg library tests, 89 doctests, formatting, diff checks, and all-feature library Clippy pass.