feat: foreach-style enumerator for top-level groups (#156)

[pedrosakuma]

Closes #156.

Summary

Each top-level group on {Msg}DataReader now exposes a property returning a ref struct enumerator usable with foreach — zero allocation, no closure capture, ref readonly Current for zero-copy access.

if (OrderBookData.TryParse(buffer, out var reader))
{
    foreach (ref readonly var bid in reader.Bids) { /* ... */ }
    foreach (ref readonly var ask in reader.Asks) { /* ... */ }
}

Design — independent per-group views

Each property access creates a fresh enumerator that computes its own start offset by chaining static Skip{Group} helpers across prior groups. This avoids the footgun of a shared cursor:

• ✅ Out-of-order access (Asks before Bids) works
• ✅ Early break does not corrupt later groups
• ✅ Repeated iteration of the same group works (each access = fresh enumerator)
• ✅ ReadGroups continues to work unchanged (idempotent)
• ✅ Bounds checks in MoveNext and Skip helpers — malformed buffers return false instead of throwing

Gating

Emitted only when all top-level groups are simple (no nested groups, no group-level varData). Complex messages keep ReadGroups only — the foreach API is purely additive.

Tests

6 new integration tests in GroupForeachEnumeratorTests.cs:

• In-order iteration with multiple entries per group
• Out-of-order access (Asks before Bids)
• Repeated iteration produces consistent results
• Empty groups yield nothing
• Early break does not corrupt the next group
• BytesConsumed is unchanged by foreach (still managed exclusively by ReadGroups)

Full suite: 187 unit + 144 integration tests green.