refactor: seal marker, smooth, scan_trial to pub(crate)

The #[doc(hidden)] pub mod hatch in lib.rs exposes 19 internal modules
to external callers who can reach them via mozjpeg_rs::foo::bar despite
the docs.rs hiding. That's intentional for modules like dct, consts,
trellis, quant, huffman, progressive — internal benches/examples/tests
and the sibling zenjpeg crate all import from them.

But three modules have zero external reach in benches, examples, tests,
or the sibling zenjpeg tree:

- marker (JPEG marker writer, 756 LOC)
- smooth (input smoothing filter, 227 LOC)
- scan_trial (sequential scan trial encoder, 443 LOC)

Seal those three to pub(crate) so they can't drift into the hidden
API surface. Keeps the #[allow(dead_code)] attribute on each — sealing
exposed some pre-existing dead code (MarkerWriter methods, scan_trial
bookkeeping fields) that's a separate cleanup concern.

Also delete MarkerWriter::get_ref — the one dead item with no callers
anywhere in the crate, found while chasing the sealing warnings.

types is NOT sealed in this commit: it exposes several pub items
(ColorSpace, CompressionProfile, DctMethod, ScanInfo, ComponentInfo,
QuantTable, HuffmanTable, DctBlock, SampleBlock, FloatBlock) that are
not re-exported at the crate root, so sealing could theoretically
break an external user reaching them via mozjpeg_rs::types::X. Left
for a follow-up that also triages the extra items.

cargo test --lib: 292 passed.

Author: lilith

src/lib.rs

@@ -179,9 +179,8 @@ pub mod fast_entropy;
 pub mod huffman;
 
 /// JPEG marker writing (internal).
-#[doc(hidden)]
 #[allow(dead_code)]
-pub mod marker;
+pub(crate) mod marker;
 
 /// Progressive scan generation (internal).
 #[doc(hidden)]
@@ -199,19 +198,17 @@ pub mod quant;
 pub mod sample;
 
 /// Input smoothing filter (internal).
-#[doc(hidden)]
 #[allow(dead_code)]
-pub mod smooth;
+pub(crate) mod smooth;
 
 /// Scan optimization (internal).
 #[doc(hidden)]
 #[allow(dead_code)]
 pub mod scan_optimize;
 
 /// Sequential scan trial encoding (internal).
-#[doc(hidden)]
 #[allow(dead_code)]
-pub mod scan_trial;
+pub(crate) mod scan_trial;
 
 /// SIMD acceleration (internal).
 #[doc(hidden)]

src/marker.rs

@@ -436,11 +436,6 @@ impl<W: Write> MarkerWriter<W> {
         self.output
     }
 
-    /// Get a reference to the underlying output.
-    pub fn get_ref(&self) -> &W {
-        &self.output
-    }
-
     /// Get a mutable reference to the underlying output.
     pub fn get_mut(&mut self) -> &mut W {
         &mut self.output