Improve dashboard context telemetry and PNG export speed

[Alek99]

Summary

• make the 10/20/50-chart dashboard benchmark retain startup, heap, redraw, and scroll metrics when Chrome evicts contexts
• record per-chart webglcontextlost / webglcontextrestored events, phases, initial/scrolled nonblank IDs, active chart count, and a stable loss-free ceiling
• fix the interaction benchmark warm-up by synchronously completing GPU work before samples
• lower native PNG zlib compression from level 9 to level 6, with regression coverage for indexed and truecolor output

Why

Chrome accepts contexts beyond its per-page quota by evicting older contexts. The previous dashboard probe inferred the cap from blank pixels, threw on the first partial dashboard, and discarded the scaling metrics needed to design shared-context rendering. It also could not distinguish LRU eviction from allocation failure.

The interaction probe issued a warm draw without a readback, so its first timed wheel gesture could pay deferred shader/GPU work. Native PNG export also spent most of its time in level-9 deflate for a very small size win.

Measured impact

• software GL: 20 charts retain metrics with 4 loss events and 16 active charts; 50 charts retain metrics with 34 losses and 16 active charts
• hardware reproduces the same context cap while rendering 10/20/50 charts in roughly 86/138/285 ms
• 1M-line native PNG median improves from about 298 ms to about 64 ms; output grows about 2.65%
• the 12-repetition software interaction stress probe passes all six chart families after the warm-up fix

Validation

• cargo build --release
• .venv/bin/python -m pytest -q (880 passed)
• .venv/bin/ruff check .
• .venv/bin/ruff format --check on all changed Python files
• .venv/bin/python scripts/verify_local.py --only benchmark_harness
• software-GL dashboard 10/20/50 report plus verify_benchmark_report.py --kind dashboard-browser
• scripts/interaction_stress_smoke.py with 12 repetitions
• standard workflow benchmark with 1M-line native PNG export

Repository-wide ruff format --check . currently flags upstream examples/demo.ipynb, which this branch does not modify.