[refactor](trx-wxsat): unify image encoding to shared PNG module

Extract common image_enc module at crate root with encode_grayscale_png
and encode_rgb_png helpers. Both NOAA APT and Meteor-M LRPT now use PNG
as the output format through the shared encoder. Drop jpeg image feature
dependency.

https://claude.ai/code/session_01JA13DHuzuHUL4nSBBRU83f
Signed-off-by: Claude <noreply@anthropic.com>

Cargo.lock

@@ -1261,7 +1261,6 @@ dependencies = [
  "bytemuck",
  "byteorder",
  "color_quant",
- "jpeg-decoder",
  "num-traits",
  "png",
 ]
@@ -1341,12 +1340,6 @@ dependencies = [
  "libc",
 ]
 
-[[package]]
-name = "jpeg-decoder"
-version = "0.3.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "00810f1d8b74be64b13dbf3db89ac67740615d6c891f0e7b6179326533011a07"
-
 [[package]]
 name = "js-sys"
 version = "0.3.85"