Add live scheduler timeline

[espmaniac]

What changed

• add a live Scheduler timeline below the Tasks table
• keep task labels fixed while execution history enters from the right and moves left
• show one color-coded lane per task with measured scheduler-slice durations
• calculate each task's execution-time share for the visible window
• show exact slice duration and age on hover
• adapt the rolling window to low fuel rates (for example, 40 seconds at 1 fuel/ms)
• expose a bounded browser-runtime trace ring buffer and batch-read API
• preserve trace history while fuel is paused and handle cached runtimes, narrow screens, and reduced motion

Why

The task state is normally sampled between scheduler slices, so CPU-bound tasks often appear as Ready even though they are running fairly. A real scheduler trace makes execution order, frequency, and relative time visible without falsifying task state.

Validation

• GitHub Actions Emscripten build: passed
• two spin_forever.wasm tasks: two live lanes, alternating slices, approximately 47% / 53% execution share
• 1 fuel/ms: adaptive window expands to 40 seconds
• 0 fuel/ms: status changes to Fuel paused while the prior window is retained
• runtime reports Runtime: OK
• no page overflow in the public preview
• node --check main.js
• bash -n build-browser.sh
• HTML parser smoke check
• git diff --check
• C11 syntax check with -Wall -Wextra -Werror