build(android): split GPU acceleration into optional features

Both `kon-transcription` and `kon-llm` previously hardcoded their native
acceleration features in Cargo.toml — `whisper-rs` with `vulkan`,
`llama-cpp-2` with `openmp` + `vulkan`. That worked everywhere desktop
ships (Linux/macOS/Windows all have Vulkan via MoltenVK on Mac), but it
made an Android build structurally impossible: NDK builds against drivers
that vary wildly across SoCs (Adreno OK, Mali patchy, PowerVR worse), and
some older devices have no Vulkan at all.

Roadmap step 0 from the Android plan: make the GPU acceleration
opt-in so a CPU-only target compiles. Reuses the existing pattern that
README's "future Windows non-AVX2 build" comment hinted at.

- kon-transcription: new `whisper-vulkan` feature gates `whisper-rs/vulkan`
  via the optional-syntax `whisper-rs?/vulkan`. Default features stay as
  `["whisper", "whisper-vulkan"]` so desktop is unchanged.
- kon-llm: new `gpu-vulkan` and `openmp` features each gate the matching
  `llama-cpp-2` feature. Default stays `["gpu-vulkan", "openmp"]`. They are
  independent so an Android Vulkan build can opt into vulkan without
  openmp (NDK OpenMP linking has known cross-version fragility).

CPU-only build invocations:
  cargo build -p kon-transcription --no-default-features --features whisper
  cargo build -p kon-llm --no-default-features

Verified: all 91 tests in the buildable-in-sandbox crates still pass.
The two crates whose Cargo.toml changed (kon-transcription, kon-llm)
can't be compiled in this sandbox (ort-sys CDN + cmake-built llama.cpp);
CI's Linux/macOS/Windows builders will exercise the default-feature path
exactly as before.

https://claude.ai/code/session_0189xUb6ie6t9qHkzatGZ9Rb

crates/llm/Cargo.toml

@@ -3,11 +3,22 @@ name = "kon-llm"
 version = "0.1.0"
 edition = "2021"
 
+[features]
+# Default desktop build keeps the existing openmp + vulkan acceleration.
+# Mobile / CPU-only targets can drop one or both via:
+#   cargo build -p kon-llm --no-default-features
+# These are independent so an Android Vulkan build can opt into vulkan
+# without openmp (the NDK ships OpenMP libs but the toolchain configuration
+# is fragile across NDK versions).
+default = ["gpu-vulkan", "openmp"]
+gpu-vulkan = ["llama-cpp-2/vulkan"]
+openmp = ["llama-cpp-2/openmp"]
+
 [dependencies]
 kon-core = { path = "../core" }
 encoding_rs = "0.8"
 futures-util = "0.3"
-llama-cpp-2 = { version = "0.1.144", default-features = false, features = ["openmp", "vulkan"] }
+llama-cpp-2 = { version = "0.1.144", default-features = false }
 num_cpus = "1"
 reqwest = { version = "0.12", default-features = false, features = ["rustls-tls", "stream"] }
 serde = { version = "1", features = ["derive"] }