feat: direct varData properties (#162) + foreach vs callback bench (#156)

[pedrosakuma]

Summary

Two related changes around the v1.5.0 enumerator work, shipped together as v1.6.0.

feat: direct varData properties on {Msg}DataReader (#162)

Expose top-level <data> fields as zero-allocation direct properties alongside ReadGroups. Each property recomputes its offset by chaining the existing Skip{Group} helpers (O(1) per simple group) plus per-data SkipData{Prev} helpers — fully stateless, safe regardless of access order, no closure capture.

if (NewOrderData.TryParse(buffer, out var reader))
{
    foreach (ref readonly var leg in reader.Legs) { /* ... */ }
    var coid = reader.ClientOrderId.VarData;   // ReadOnlySpan<byte>, zero-alloc
}

Gating (same as #156 enumerators): all top-level groups must be simple (no nested groups, no group-level varData). Also valid when the message has only varData. When a varData name collides with a reserved member of the reader struct (e.g. <data name="data"> clashing with the Data block accessor) the direct property is silently skipped; ReadGroups remains usable.

For repeated reads, cache the returned value locally (documented in the XML doc on the property).

bench: foreach vs callback group decode (#156)

Adds a benchmark validating the perf premise of the v1.5.0 foreach enumerator API.

Tests

• 336 tests passing (187 unit + 149 integration, +2 new for direct varData properties).
• New tests cover: direct property access matches callback bytes, stateless across multiple accesses, works before/after group iteration, works with empty group + varData.

Release

• Bumps version to 1.6.0.
• CHANGELOG entry under [1.6.0] - 2026-04-29.
• README updated with new example.