Clean up pre-existing deprecation warnings (matplotlib scatter marker, plotly kaleido engine)

These two deprecation warnings surfaced during the full re-execution validation for the anaconda=2026.06 upgrade (#346). They are pre-existing (not caused by that upgrade) and non-blocking — the affected lectures still build — but worth cleaning up, especially the matplotlib one, which becomes a hard error in a future release.

1. Matplotlib — positional marker in scatter() (will error in matplotlib 3.12)

lectures/chang_credible.md:849

ax.scatter(R[0], R[1], 150, 'black', 'o', zorder=1)

MatplotlibDeprecationWarning: Passing the marker parameter of scatter() positionally is deprecated since Matplotlib 3.10; the parameter will become keyword-only in 3.12.

Suggested fix — pass the size/color/marker as keywords:

ax.scatter(R[0], R[1], s=150, c='black', marker='o', zorder=1)

2. plotly to_image(..., engine="kaleido") deprecation

DeprecationWarning: Support for the 'engine' argument is deprecated and will be removed after September 2025. Kaleido will be the only supported engine at that time.

Kaleido is already the default/only engine going forward, so the engine="kaleido" argument can simply be dropped (several calls in knowing_forecasts_of_others.md already omit it, e.g. fig2.to_image(format="png")).

Affected lines:

• lectures/BCG_complete_mkts.md:1197
• lectures/BCG_incomplete_mkts.md:1273, :1637, :1688, :1749, :1779
• lectures/knowing_forecasts_of_others.md:1024

# from
Image(fig.to_image(format="png", engine="kaleido"))
# to
Image(fig.to_image(format="png"))

Tasks

• chang_credible: pass s / c / marker as keywords in ax.scatter(...)
• BCG_complete_mkts, BCG_incomplete_mkts, knowing_forecasts_of_others: drop engine="kaleido" from to_image(...) calls

Found during validation in #346.

Review the listed lines in lectures/chang_credible.md, BCG_complete_mkts.md, BCG_incomplete_mkts.md, and knowing_forecasts_of_others.md. Start by comparing the affected calls with the warning text and the existing warning-free to_image example. Done means the keyword and engine deprecation warnings no longer appear during full re-execution validation.