vtracer-bench: codec-free library, codecs behind a cli feature

- The library now speaks the image crate's buffer types only:
  fidelity(&RgbImage, &RgbImage, thresh) -> (FidelityReport, GrayImage).
  A tracer embedding the metric pulls no image codec at all.
- New `cli` feature (default on) brings png/jpeg/webp decoding and
  gates the vtracer-bench bin (required-features).
- dssim-core with default features off: drops the rayon thread pool.
- Scores are unchanged: the CLI prints the same value before and after.

Cargo.lock

@@ -194,31 +194,6 @@ dependencies = [
  "cfg-if",
 ]
 
-[[package]]
-name = "crossbeam-deque"
-version = "0.8.7"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5181e0de7b61eb03a81e347d6dd8797bae9da5146707b51077e2d71a54ec0ceb"
-dependencies = [
- "crossbeam-epoch",
- "crossbeam-utils",
-]
-
-[[package]]
-name = "crossbeam-epoch"
-version = "0.9.20"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2d6914041f254d6e9176c01941b21115dcfb7089e55135a35411081bd106ef3f"
-dependencies = [
- "crossbeam-utils",
-]
-
-[[package]]
-name = "crossbeam-utils"
-version = "0.8.22"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "61803da095bee82a81bb1a452ecc25d3b2f1416d1897eb86430c6159ef717c17"
-
 [[package]]
 name = "crunchy"
 version = "0.2.4"
@@ -239,7 +214,6 @@ checksum = "e3c601412450ff29a9258b2f85b18b38f658caf70fad1692f40ca863d86cb753"
 dependencies = [
  "imgref",
  "itertools 0.14.0",
- "rayon",
  "rgb",
 ]
 
@@ -645,26 +619,6 @@ dependencies = [
  "proc-macro2",
 ]
 
-[[package]]
-name = "rayon"
-version = "1.12.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "fb39b166781f92d482534ef4b4b1b2568f42613b53e5b6c160e24cfbfa30926d"
-dependencies = [
- "either",
- "rayon-core",
-]
-
-[[package]]
-name = "rayon-core"
-version = "1.13.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "22e18b0f0062d30d4230b2e85ff77fdfe4326feb054b9783a3460d8435c8ab91"
-dependencies = [
- "crossbeam-deque",
- "crossbeam-utils",
-]
-
 [[package]]
 name = "resvg"
 version = "0.45.1"
@@ -994,7 +948,7 @@ dependencies = [
 
 [[package]]
 name = "vtracer-bench"
-version = "1.0.0-alpha.4"
+version = "1.0.0-alpha.5"
 dependencies = [
  "dssim-core",
  "image",