c/BUILD: add public shared-library target (libLiteRtLm.so / .dylib / .dll)

[DenisovAV]

Summary

Adds a cc_binary(linkshared=True, name="LiteRtLm") target to c/BUILD that produces a flat shared library with only LiteRt* / litert_lm_* symbols exported. This unblocks every cross-language consumer of LiteRT-LM that wants to ship through a native FFI (Dart, Python, Node, Rust, .NET) without pulling Bazel into their build pipeline.

Closes #2154.

What's added

Three files, all under c/:

1. c/BUILD — new `cc_binary` target named `LiteRtLm`. Bazel adds the platform-specific suffix automatically:
   • `libLiteRtLm.so` on Linux / Android
   • `libLiteRtLm.dylib` on macOS / iOS
   • `LiteRtLm.dll` on Windows
2. `c/dynamic_list.lds` — Linux linker dynamic-list. Extends the default visibility set with `LiteRt*` / `litert_lm_*` so plugin-internal `dlsym(RTLD_DEFAULT)` lookups (e.g. WebGPU accelerator auto-registration) keep resolving — important: it does NOT replace the default set, so absl/protobuf internals stay visible to internal-internal traffic but not to external consumers (consumers see only the C API).
3. `c/windows_exports.def` — Windows DLL whitelist. One entry per public C API function; new C API additions to `engine.h` need a corresponding entry here or the build fails with LNK2019.

Per-platform export mechanism in the `linkopts` select:

Platform	Mechanism	Notes
macOS / iOS	`-Wl,-exported_symbol,_LiteRt*` + `litert_lm*`	Two glob patterns, matches the C API surface
Linux	`-Wl,--dynamic-list=...` + `.lds`	Extends visibility, doesn't replace
Windows	`win_def_file` attribute	Bazel native; `/DEF:` in linkopts gets clobbered by the MSVC link.exe action

What's NOT changed

• Existing `:engine` and `:engine_cpu` cc_library targets — unchanged.
• C API surface — no new functions; everything in the .def / dynamic_list is already in `engine.h`.
• Android NDK build — still uses its own `runtime/engine/litert_lm_android` path.

Test plan

• flutter_gemma carries this exact pattern downstream since v0.14.0 (`patch_c_api.sh` section 1) on all five platforms; the produced `libLiteRtLm.so` / `.dylib` / `.dll` work for Dart FFI `DynamicLibrary.open` + the C API symbol surface.
• WebGPU accelerator plugin auto-registration works on Linux with `--dynamic-list` (it relies on `dlsym(RTLD_DEFAULT)` resolving `LiteRt*` symbols against the loaded `libLiteRtLm.so` because `Dart's DynamicLibrary.open` uses RTLD_LOCAL — verified on Linux T4 + L4 GPU VMs).
• App Store distribution works on iOS/macOS — symbol whitelist keeps `Frameworks/` clean.
• Upstream CI on this branch.

Cross-references

• Apple Mach-O bundling: companion dylibs lack -headerpad_max_install_names; gpu_registry/sampler_factory dlopen by basename incompatible with .framework bundling (App Store ITMS-90432) #2151 — Apple Mach-O umbrella (this PR is independent of Apple Mach-O bundling: companion dylibs lack -headerpad_max_install_names; gpu_registry/sampler_factory dlopen by basename incompatible with .framework bundling (App Store ITMS-90432) #2151)
• c/engine.cc: propagate cache_dir to vision/audio executor settings #2153 — separate small fix (cache_dir vision/audio propagation)

CLA

Will sign Individual CLA before merge if needed; flagging upfront.