feat: add lucy-vton-3.6 realtime model at 720p

[tomershlasky]

Description

Registers lucy-vton-3.6 on the realtime and video surfaces. Mirrors DecartAI/sdk#217 — the JS SDK registry is the source of truth this package's tests are written against. The model is already served in production; this only makes it reachable through the typed SDK surface.

1. lucy-vton-latest still resolves to lucy-vton-3.5. 3.6 is opt-in by explicit name; the alias comments are untouched.
2. 3.6 is not offered on the fast tier, so the entry omits the speed capability and the fast-mode prose is unchanged.
3. lucy-vton-3.5 is unchanged — purely additive.

1280x720 at 30 fps, confirmed against the deployed model. As with 3.5, this SDK is realtime-only, so there is no batch entry.

Usage

auto model = decart::models::realtime("lucy-vton-3.6");
// urlPath "/v1/stream", 1280x720 @ 30fps, supportedSpeeds empty

Changes

File	Change
src/models.cpp	kRealtime entry, and the std::array<Entry, N> size literal 7 to 8
include/decart/models.h	the accepted-canonical-names doc comment on realtime()
tests/test_models.cpp	geometry block asserting supportedSpeeds.empty(), a negative in the fast-capability case, and canonicalOnly count 4 to 5

The array size literal is the one easy-to-miss edit, but it is a hard compile error if forgotten, so it is self-catching.

Why the speed capability is omitted

supportedSpeeds is a claim that the service will serve the model on that tier and price it accordingly — an unsupported tier is rejected at connect time, not silently ignored. 3.6 is not offered on it, so the capability is left off here and the fast-model pin in the tests is deliberately unchanged.

Verification

Built and run locally on Linux / GCC 11.5 / CMake 3.22:

Command	Result
cmake --build build	100% built
ctest --test-dir build --output-on-failure	1/1 passed
./decart_tests	19 test cases, 97 assertions, 0 failed
clang-format --dry-run --Werror on the changed files	clean

Built with -DDECART_BUILD_REALTIME=OFF because the LiveKit C++ SDK release download is not reachable from this environment; that flag excludes the transport only, and src/models.cpp plus all of tests/test_models.cpp are in the core library and fully exercised. CI builds the realtime module on all three platforms.

listRealtime() honors canonicalOnly still passes with 3.6 in the registry — that is the real proof the array size and canonical flag are right.

Compatibility

Source- and ABI-compatible. kRealtime is a file-local constexpr in an anonymous namespace, so growing it changes no exported symbol, type layout or function signature.