feat(audio): JACK backend foundation (#263 Tier 3 — player wiring pending)

Backend-side foundation for a native JACK output. The hard, RT-sensitive part
is done and compiles: jack_backend.rs holds a JackStream that opens a JACK client
'qbz' with stable ports qbz:out_FL/FR, a lock-free SPSC ring buffer, and an
allocation-free process callback (de-interleaves stereo f32 to the ports). The
feeder pushes interleaved f32 via write_f32().

Also: jack 0.13 dep (linux), AudioBackendType::Jack + its factory arm + the
qbz-slint label/bit-perfect match arms. NOT yet offered in the selector
(available_backends push commented) because the player wiring is still pending:
StreamType::Jack + the dispatch branch (backend==Jack -> JackStream) + a
PlaybackEngine::Jack feeder thread that resamples each track to the JACK graph
rate. JACK mode will be opt-in and NOT bit-perfect (one fixed graph rate); the
ALSA-exclusive / DAC-passthrough bit-perfect paths are untouched.

TODO(distribution): feature-gate jack for Flatpak/distro builds without libjack.

cc

crates/Cargo.lock

@@ -1037,7 +1037,7 @@ checksum = "0b023947811758c97c59bf9d1c188fd619ad4718dcaa767947df1cadb14f39f4"
 dependencies = [
  "glob",
  "libc",
- "libloading",
+ "libloading 0.8.9",
 ]
 
 [[package]]
@@ -1601,7 +1601,7 @@ version = "0.5.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ab8ecd87370524b461f8557c119c405552c396ed91fc0a8eec68679eab26f94a"
 dependencies = [
- "libloading",
+ "libloading 0.8.9",
 ]
 
 [[package]]
@@ -2328,7 +2328,7 @@ dependencies = [
  "glutin_egl_sys",
  "glutin_glx_sys",
  "glutin_wgl_sys",
- "libloading",
+ "libloading 0.8.9",
  "objc2 0.6.4",
  "objc2-app-kit 0.3.2",
  "objc2-core-foundation",
@@ -3260,6 +3260,33 @@ version = "1.0.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "92ecc6618181def0457392ccd0ee51198e065e016d1d527a7ac1b6dc7c1f09d2"
 
+[[package]]
+name = "jack"
+version = "0.13.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f7811b07bcac5dafabf814ab52c4b0ca9b7948aa1e279f572f03aa6544d47d27"
+dependencies = [
+ "bitflags 2.11.0",
+ "jack-sys",
+ "lazy_static",
+ "libc",
+ "log",
+]
+
+[[package]]
+name = "jack-sys"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6013b7619b95a22b576dfb43296faa4ecbe40abbdb97dfd22ead520775fc86ab"
+dependencies = [
+ "bitflags 1.3.2",
+ "lazy_static",
+ "libc",
+ "libloading 0.7.4",
+ "log",
+ "pkg-config",
+]
+
 [[package]]
 name = "jiff"
 version = "0.2.25"
@@ -3460,6 +3487,16 @@ dependencies = [
  "cc",
 ]
 
+[[package]]
+name = "libloading"
+version = "0.7.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b67380fd3b2fbe7527a606e18729d21c6f3951633d0500574c4dc22d2d638b9f"
+dependencies = [
+ "cfg-if",
+ "winapi",
+]
+
 [[package]]
 name = "libloading"
 version = "0.8.9"
@@ -4991,6 +5028,7 @@ dependencies = [
  "coreaudio-rs",
  "dirs 6.0.0",
  "ebur128",
+ "jack",
  "log",
  "objc2-core-audio",
  "qbz-models",
@@ -7096,7 +7134,7 @@ checksum = "a90a0ca3ee6a69f2ad28fd11621a4c3f03b371f366be500b64df260c4ffbafb4"
 dependencies = [
  "as-raw-xcb-connection",
  "ctor",
- "libloading",
+ "libloading 0.8.9",
  "pkg-config",
  "tracing",
 ]
@@ -8014,6 +8052,22 @@ dependencies = [
  "rustix 0.38.44",
 ]
 
+[[package]]
+name = "winapi"
+version = "0.3.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5c839a674fcd7a98952e593242ea400abe93992746761e38641405d28b00f419"
+dependencies = [
+ "winapi-i686-pc-windows-gnu",
+ "winapi-x86_64-pc-windows-gnu",
+]
+
+[[package]]
+name = "winapi-i686-pc-windows-gnu"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ac3b87c63620426dd9b991e5ce0329eff545bccbbb34f3be09ff6fb6ab51b7b6"
+
 [[package]]
 name = "winapi-util"
 version = "0.1.11"
@@ -8023,6 +8077,12 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "winapi-x86_64-pc-windows-gnu"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"
+
 [[package]]
 name = "windows"
 version = "0.61.3"
@@ -8726,7 +8786,7 @@ dependencies = [
  "as-raw-xcb-connection",
  "gethostname",
  "libc",
- "libloading",
+ "libloading 0.8.9",
  "once_cell",
  "rustix 1.1.4",
  "x11rb-protocol",