feat(background-task): rendu live des tâches de fond — subscriber UI + canal IPC (#58)

Câble un canal attachable au flux de sortie d'une tâche de fond (runner
infrastructure + commande app-tauri + port/adaptateurs frontend) et le
panneau ProjectWorkStatePanel s'y abonne pour un rendu live au lieu d'un
état figé au dernier snapshot.

Validations obtenues avant commit :
- cargo test -p infrastructure --test background_task_runner : vert
- cargo check -p backend -p app-tauri : vert
- npx vitest run src/features/workstate/workstate.test.tsx : vert
- npm run typecheck : vert
- verdict QA #58 : vert

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Cargo.lock

@@ -1192,6 +1192,16 @@ dependencies = [
  "percent-encoding",
 ]
 
+[[package]]
+name = "fs4"
+version = "0.12.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c29c30684418547d476f0b48e84f4821639119c483b1eccd566c8cd0cd05f521"
+dependencies = [
+ "rustix",
+ "windows-sys 0.52.0",
+]
+
 [[package]]
 name = "fsevent-sys"
 version = "4.1.0"
@@ -1970,6 +1980,7 @@ dependencies = [
  "async-trait",
  "domain",
  "fastembed",
+ "fs4",
  "futures-util",
  "git2",
  "hex",
@@ -2290,6 +2301,12 @@ dependencies = [
  "vcpkg",
 ]
 
+[[package]]
+name = "linux-raw-sys"
+version = "0.4.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d26c52dbd32dccf2d10cac7725f8eae5296885fb5703b261f7d0a0739ec807ab"
+
 [[package]]
 name = "litemap"
 version = "0.8.2"
@@ -3526,6 +3543,19 @@ dependencies = [
  "semver",
 ]
 
+[[package]]
+name = "rustix"
+version = "0.38.44"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fdb5bc1ae2baa591800df16c9ca78619bf65c0488b41b96ccec5d11220d8c154"
+dependencies = [
+ "bitflags 2.12.1",
+ "errno",
+ "libc",
+ "linux-raw-sys",
+ "windows-sys 0.59.0",
+]
+
 [[package]]
 name = "rustls"
 version = "0.23.40"