fix(lzma2): restore Debug/Clone on Encoder; document the stub→struct break

CHANGELOG.md

@@ -29,6 +29,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- **Breaking:** `compcol::lzma2::Encoder` is now a normal (stateful) struct
+  instead of the former permanently-`Unsupported` unit-struct stub, because the
+  working encoder buffers chunk state. As a result it **no longer implements
+  `Copy`** and can no longer be constructed via a unit-struct literal; construct
+  it through `Lzma2::encoder()` as with every other codec. It still derives
+  `Debug` + `Clone`. (No effect on the decoder or any other codec.)
 - **lz5 (Lizard) Huffman sub-streams** stay `Unsupported`, now with a precise
   rationale in the module docs: the Huff0 entropy stage selects X1/X2 from
   `(regenSize, comprLen)` at runtime and there is no reference encoder or

src/lzma2/mod.rs

@@ -225,6 +225,11 @@ enum EncPhase {
 /// (dict + props + state) at the chunk boundary, emitting a compressed chunk
 /// (control `0xE0`) when that shrinks the data and an uncompressed chunk
 /// (control `0x01`) otherwise.
+///
+/// Note: unlike the former permanently-`Unsupported` stub (a unit struct),
+/// the working encoder buffers state, so it is a normal struct and is no
+/// longer `Copy` — construct it via [`Lzma2::encoder()`](crate::Algorithm).
+#[derive(Debug, Clone)]
 pub struct Encoder {
     phase: EncPhase,
     /// Staged bytes for the current chunk (or end marker), drained to the

src/lzma2_internal/lzma2_encoder.rs

@@ -103,7 +103,7 @@ const NIL: u32 = u32::MAX;
 /// raise `nice_match` (the length at which the chain walk gives up and
 /// accepts the current match). This is the same speed-vs-ratio knob that
 /// xz-utils exposes — we just expose a small subset.
-#[derive(Clone, Copy)]
+#[derive(Debug, Clone, Copy)]
 pub(crate) struct EncoderParams {
     pub max_chain: usize,
     pub nice_match: u32,