Ch1 lecture notes and labs

[ARS183]

Adds the Chapter 1 material to the book: the lecture-note pages reproduced
from the LaTeX source, and the two Friday computer labs that go with them.

Lecture notes — intro, sums_series_approx, coord_sys, gradient,
divergence, curl.

Labs — labs/week01_series_grad.md (series and the gradient) and
labs/week02_div_curl.md (divergence, curl, and a formative assessment on
both chapters). Students fill ___ blanks; each task states a physical
question and ends with a self-check that reads what they wrote. Plotting and
quadrature helpers live in labs/fwtools.py beside the pages.

Both lab pages set execution_mode: 'off' in their front matter, since the
task cells contain blanks by design. They are meant to be run with Live Code,
which resolves plotly and scipy at runtime, so requirements.txt is unchanged.

Every task was executed end to end with the solutions filled in before this
was opened.