feat(compose-witness): Wasm-component PoC for REQ-086 witness MC/DC

Cargo.toml

@@ -5,6 +5,14 @@ members = [
     "rivet-cli",
     "etch",
 ]
+# compose-witness is a sibling cargo project, NOT a workspace member: it
+# builds to a Wasm component (cdylib + wit-bindgen-rt) targeting
+# wasm32-wasip1, has its own lockfile, and is built by either
+# `cargo component build --manifest-path compose-witness/Cargo.toml` or
+# the Bazel `rules_wasm_component` pipeline (compose-witness/BUILD.bazel,
+# REQ-086). Excluding it keeps host `cargo --workspace` / `cargo test
+# --all` clean of wasm-only crate-types they cannot compile.
+exclude = ["compose-witness"]
 
 [workspace.package]
 version = "0.12.0"

MODULE.bazel

@@ -79,3 +79,11 @@ rocq.toolchain(
 )
 use_repo(rocq, "rocq_toolchains", "rocq_stdlib")
 register_toolchains("@rocq_toolchains//:all")
+
+# ── rules_wasm_component (REQ-086 Wasm component + witness MC/DC) ──────
+#
+# Org-standard Bazel rules for WebAssembly Component Model builds.
+# Bundles the `witness` toolchain (toolchains/witness_toolchain.bzl) and
+# the `wasm_module_coverage` rule, which is how we produce MC/DC coverage
+# evidence on the feature-model composition core (REQ-086).
+bazel_dep(name = "rules_wasm_component", version = "1.0.0")

compose-witness/.gitignore

@@ -0,0 +1,7 @@
+/target/
+
+# Generated by cargo-component / wit-bindgen on every build. Keeping it
+# out of git avoids the "did you forget to regenerate after editing
+# wit/world.wit?" desync risk; reviewers reproduce it locally with
+# `cargo component build --manifest-path compose-witness/Cargo.toml`.
+/src/bindings.rs

compose-witness/BUILD.bazel

@@ -0,0 +1,56 @@
+# REQ-086 — Wasm component exposing the REQ-083 composition core's pure
+# constraint-prefixing path. The `witness` toolchain (bundled with
+# rules_wasm_component) instruments the resulting module and emits MC/DC
+# branch coverage; Rivet ingests the coverage envelope as REQ-083 /
+# REQ-086 requirement-to-test evidence.
+#
+# Canonical Bazel pipeline:
+#   wit_library  ──┐
+#                  ├─► rust_wasm_component_bindgen ──► wasm_module_coverage
+#   src/lib.rs   ──┘
+#
+# Cargo-component is supported in parallel (see compose-witness/Cargo.toml)
+# for local dev iteration, but the org-standard build path is this one —
+# it composes cleanly with the existing certification pipeline
+# (meld_fuse / wasm_attest / wasm_verify_chain) when we expand beyond a
+# single component.
+
+load("@rules_wasm_component//rust:defs.bzl", "rust_wasm_component_bindgen")
+load("@rules_wasm_component//wasm:defs.bzl", "wasm_module_coverage")
+load("@rules_wasm_component//wit:defs.bzl", "wit_library")
+
+package(default_visibility = ["//visibility:public"])
+
+# 1. The WIT interface: a single `prefix-features` export.
+wit_library(
+    name = "compose_witness_interfaces",
+    srcs = ["wit/world.wit"],
+    package_name = "pulseengine:compose-witness",
+    world = "compose-witness",
+)
+
+# 2. The Rust component itself — calls the inlined pure prefixing path.
+rust_wasm_component_bindgen(
+    name = "compose_witness",
+    srcs = ["src/lib.rs"],
+    # debug profile keeps DWARF debug info so witness's LCOV report
+    # carries source-line records (otherwise it'd be structural only —
+    # see examples/witness_example/BUILD.bazel comment block).
+    profiles = ["debug"],
+    wit = ":compose_witness_interfaces",
+)
+
+# 3. The witness MC/DC pass. The harness drives the component's
+#    `prefix-features` export — every REQ-083 fixture scenario plus the
+#    per-condition flip pairs MC/DC requires — and witness emits the
+#    LCOV truth tables / signed coverage envelope.
+#
+# NOTE: `wasm_module_coverage` operates on core modules, not components,
+# per examples/witness_example/BUILD.bazel. For a single-component
+# subject this works directly; multi-component compositions go through
+# `meld_fuse` first.
+wasm_module_coverage(
+    name = "compose_witness_coverage",
+    module = ":compose_witness",
+    invoke = ["prefix-features"],
+)

compose-witness/Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "compose-witness"
+version = "0.1.0"
+edition = "2024"
+authors = ["PulseEngine <https://github.com/pulseengine>"]
+license = "Apache-2.0"
+publish = false
+description = "Wasm-component PoC exposing the REQ-083 composition core's pure prefixing path so the pulseengine `witness` tool can measure MC/DC on it (REQ-086)."
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+wit-bindgen-rt = { version = "0.44.0", features = ["bitflags"] }
+serde = { version = "1", features = ["derive"] }
+serde_yaml = "0.9"
+
+[package.metadata.component]
+package = "pulseengine:compose-witness"
+
+[package.metadata.component.target]
+path = "wit"
+world = "compose-witness"
+
+[package.metadata.component.dependencies]