Reuse Builder buffer for zero-alloc tight-loop encoding

[khanaffan]

Summary

Adds a buffer-reuse API to msgpack::Builder so a single Builder can encode many varying blobs in a tight loop while reusing one heap allocation — no per-iteration malloc.

Motivation

Builder::build() moves the internal buffer into the returned Blob, so a Builder built and discarded each iteration allocates a fresh buffer every time. Hot serialization loops (sockets, files, DB rows) paid a malloc/free per blob.

API

Method	Description
Builder& reset() noexcept	Rewind to empty, keeping the heap allocation (capacity)
Builder& reserve(size_t bytes)	Pre-grow the buffer to skip warm-up growth
size_t capacity() const noexcept	Inspect the retained allocation

reset() is built on std::vector::clear(), which keeps capacity. Combined with the existing zero-copy buf_data()/buf_size() accessors, the loop performs zero allocations after warm-up. build() is unchanged and still hands off ownership for one-shot use.

Builder b;
b.reserve(64);
for (const auto& item : items) {
    b.reset();                       // rewind, keep the allocation
    b.map_header(2)
      .string("id").integer(item.id)
      .string("v").real(item.value);
    sink(b.buf_data(), b.buf_size()); // consume the zero-copy view
}

Also removes the dead private reserve(size_t) helper (never called) to free the name for the public capacity-reserve.

Tests

• New test_builder_buffer_reuse() proves reset() keeps capacity, reserve() grows it, reset() chains, and a 100-iteration loop reuses the same allocation (stable buf_data() pointer + capacity) while producing correct varying blobs.
• Unit tests: 320 -> 632 passing, 0 failed.
• Fuzz corpus and C++<->SQLite interop tests: green.
• Clean build under -Wall -Wextra.

Docs

• cpp/README.md: new "Buffer reuse (tight loops)" section + usage example; updated low-level accessors and Finalize notes.
• Test counts updated in cpp/README.md and root README.md.