Avoid unnecessary buffer zero-fill in Snappy decompression

## Description

Currently, Snappy decompression uses `resize(len, 0)` which zero-fills the buffer before writing. Since Snappy will overwrite the entire region on success, this memset is wasted work.

## Proposed Change

Write directly into spare capacity using `reserve()` + `spare_capacity_mut()` + `set_len()`, eliminating the unnecessary zero-fill.

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Locate the Snappy decompression implementation in the Rust source and inspect how the output buffer is currently resized and written. Change the buffer handling to use spare capacity as described, then verify that the existing decompression behavior and tests still pass without the zero-fill.

Written by the indexing model from the issue text.