Remove error-prone encoding APIs, enforce schema order at compile-time

[Copilot]

Implements PR #77 proposal: removes BeginEncoding() and public TryEncode*() helper methods that allowed encoding groups/varData in incorrect order, causing silent data corruption.

Changes

• MessageDefinition.cs: Made BeginEncoding(), TryEncode{GroupName}(), and TryEncode{VarDataName}() private

  • Now internal helpers for comprehensive TryEncode() methods only
  • Public API reduced to two safe overloads: span-based and callback-based

• Tests: Updated to use comprehensive TryEncode() API

  • Removed 3 tests for deprecated methods
  • 115 integration tests passing

• Documentation: Updated README.md and ENCODING_ORDER_FIX.md

  • Removed "Traditional API" sections
  • Documented breaking change and migration path

API

Before (Error-Prone)

orderBook.BeginEncoding(buffer, out var writer);
OrderBookData.TryEncodeBids(ref writer, bids);
OrderBookData.TryEncodeAsks(ref writer, asks);  // Easy to swap order → corrupt data

After (Compile-Time Safe)

// Span-based: simple, allocation-based
OrderBookData.TryEncode(orderBook, buffer, bids, asks, out bytesWritten);

// Callback-based: zero-allocation
OrderBookData.TryEncode(
    orderBook, buffer,
    bidCount, (i, ref item) => { /* populate */ },
    askCount, (i, ref item) => { /* populate */ },
    out bytesWritten);

Method signature enforces schema order. Wrong parameter order = compilation error.

Breaking Change

⚠️ User code calling BeginEncoding() or individual TryEncode*() methods will not compile. Migration requires switching to comprehensive TryEncode().

Original prompt

Implementar a proposta #77, removendo os demais fluxos de encoding, considerando que eles são sujeitos a erros.
Implementar o fluxo allocation free e com alocação.

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