feat: fixed-timestep update loop decoupled from render

[TommyRadan]

Summary

Implements #168. Splits engine::tick() into a fixed-step update decoupled from a variable-rate render, so deterministic gameplay, animation, and physics no longer depend on frame rate.

Changes

• core::time — fixed-delta accounting alongside the existing wall-clock delta:
  • accumulate(frame_delta_time) feeds elapsed real time into an accumulator, clamped to max_steps_per_frame steps — the spiral-of-death guard (a slow frame falls behind real time rather than queueing an unbounded backlog).
  • next_fixed_step() drains one whole fixed step; drive with while (time->next_fixed_step()) { update(); }.
  • interpolation_alpha() exposes the leftover remainder in [0, 1) for render-time smoothing between simulated states.
  • fixed_delta_time() accessor; rate + clamp are constructor parameters (default 60 Hz, 5 steps).
• runtime::engine::tick — pumps input once per rendered frame, drains the accumulator emitting core::frame per fixed step (game logic), then propagates scene-graph transforms and renders once. render_* events still fire per render inside renderer->render().
• window::tick — no longer emits core::frame; it now only pumps OS input at the variable render rate.

Where update vs. render fire

• Fixed step: core::frame (game-logic update). Carries the fixed delta, so per-second motion is preserved across frame rates — behaviour-preserving for the existing demos.
• Per render: input pump, scenes->update() (transform propagation), and the render_scene / render_ui / render_debug events.

Tests

• New tests/core/time_test.cpp covers draining, remainder carry-over across frames, interpolation_alpha bounds, and the spiral-of-death clamp. core/time.cpp wired into the headless test target.
• clang-format and clang-tidy naming gates pass; Release build + the 7 new tests pass locally.

Closes #168

[github-actions]

CI build artifacts

Windows Debug and Release builds are attached to the workflow run summary under Artifacts:

• AlphaEngine-windows-Debug — Debug AlphaEngine.exe + Khronos validation layer
• AlphaEngine-windows-Release — Release AlphaEngine.exe

Run #254 — commit 9c7be94.