feat(rendering): §11 transparency surface + IccBackend trait with optional lcms2 backend

Cargo.toml

@@ -26,11 +26,13 @@
     # WASM-only runtime deps (gated on `wasm` feature)
     "getrandom_02",
     "web-sys",
+    # Press-grade ICC backend (gated on `icc-lcms2` feature)
+    "lcms2",
 ]
 ignored-paths = [
     # Runnable examples — not declared as [[example]] targets because they use
     # `cargo run --manifest-path examples/rust/.../Cargo.toml` pattern.
     "examples/rust/*/main.rs",
     # Placeholder modules kept disabled for future work (see src/pipeline/mod.rs)
     "src/pipeline/input_parsers/mod.rs",
     "src/structure/builder.rs",
@@ -214,11 +216,20 @@
 # Parallel extraction
 rayon = { version = "1.12", optional = true }
 
-# ICC colour management (Firefox's qcms, pure Rust). Default features
-# bring in `cmyk` + `iccv4-enabled` which cover every colour space PDF
-# /ICCBased streams actually use.
+# ICC colour management — pure-Rust qcms (Firefox's colour engine).
+# Default features bring in `cmyk` + `iccv4-enabled` which cover every
+# colour space PDF /ICCBased streams actually use.  Gated on
+# `icc-qcms`; the legacy `icc` feature aliases to it.
 qcms = { version = "0.3", optional = true }
 
+# ICC colour management — press-grade lcms2 (Little CMS, C). Opt-in
+# behind `icc-lcms2` because it pulls in the lcms2-sys C dependency.
+# Required for CMYK→CMYK profile retargeting, Black Point Compensation,
+# and per-intent dispatch — the bits qcms 0.3 doesn't implement. Real
+# prepress / packaging consumers turn this on; library embedders
+# targeting WASM or C# AOT stay on the qcms default.
+lcms2 = { version = "6", optional = true }
+
 # Math and algorithms
 ndarray = { version = "0.17", optional = true, features = ["std"] }
 linfa = { version = "0.8", optional = true }
@@ -331,12 +342,28 @@
 # Combining `fips` with `legacy-crypto` is a compile-time error.
 fips = ["dep:aws-lc-rs", "aws-lc-rs/fips"]
 
-# Real ICC colour management via qcms (Firefox's colour engine).
-# When enabled, ICCBased colour spaces and CMYK images are converted
-# using the embedded profile instead of ISO 32000-1 §10.3.5's
-# additive-clamp fallback. Pure Rust, so WASM and C# AOT targets keep
-# working without a C toolchain dependency.
-icc = ["dep:qcms"]
+# Real ICC colour management.
+#
+# Three modes:
+#
+#  - `icc-qcms` (default, via the `icc` alias): pure-Rust Firefox-derived
+#    colour engine. CMYK→RGB through the embedded profile, no CMYK→CMYK
+#    retargeting, no Black Point Compensation. Pure Rust so WASM and
+#    C# AOT targets keep working without a C toolchain dependency.
+#
+#  - `icc-lcms2`: opt-in C-dep press-grade CMM. Adds CMYK→CMYK profile
+#    retargeting through Lab PCS (closes the DeviceN /Process ICC
+#    profile-mismatch gap for prepress/packaging workloads), BPC, and
+#    per-intent dispatch the spec asks for. Drags in lcms2-sys (C).
+#
+#  - both: when both `icc-qcms` and `icc-lcms2` are enabled the active
+#    backend is lcms2 — it's the strict superset.
+#
+# Backwards-compat alias: `icc` resolves to `icc-qcms`. Users who built
+# with `--features icc` keep the same behaviour and the same dep graph.
+icc = ["icc-qcms"]
+icc-qcms = ["dep:qcms"]
+icc-lcms2 = ["dep:lcms2"]
 
 # Gate MD5 key-derivation (R≤4 PDF Standard Security) and RC4 cipher
 # behind this feature. Default-on so existing users keep full R≤4 read/write