Optimize safe encode/decode hot paths

[Copilot]

Two targeted improvements to the safe (default) code paths, each eliminating unnecessary work from hot loops.

• Safe decompression hot loop: direct offset read — Replace read_u16(input, &mut literal_length.clone())? with u16::from_le_bytes([input[literal_length], input[literal_length + 1]]). Since input is &[u8; 16] and literal_length is 0–14 (enforced by does_token_fit), this eliminates a clone, function call, .get() bounds check, and an unreachable ? error path from the innermost loop.

• Safe compression literal copy: enable actual wild copy — The safe copy_literals_wild was passing an exactly-sized slice, defeating the wild copy optimization entirely. Now passes len + 8 bytes so slice_copy hits efficient fixed-size double_copy_trick paths for small literals, matching what the unsafe path already does. Bounds safety: MFLIMIT guarantees ≥12 bytes of input headroom; get_maximum_output_size margin covers output.

// Before (decompress hot loop) — clone + function call + error path
let offset = read_u16(input, &mut literal_length.clone())? as usize;

// After — direct indexing, provably in bounds
let offset = u16::from_le_bytes([input[literal_length], input[literal_length + 1]]) as usize;

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