Add the ability to get and set whether a slide is hidden from the
presentation. Maps to the `show` attribute on the `<p:sld>` element,
where show="0" means the slide is hidden.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add begin/end arrowhead properties to LineFormat for controlling arrow
styles on lines and connectors. Introduces MSO_LINE_END_TYPE and
MSO_LINE_END_SIZE enums and models CT_LineEndProperties for the
a:headEnd and a:tailEnd XML elements.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

feature: add Slide.is_hidden read/write property

feature: add arrowhead properties to LineFormat

Add line cap style (flat, round, square) and join style (round, bevel,
miter) properties to LineFormat. Introduces MSO_LINE_CAP_STYLE and
MSO_LINE_JOIN_STYLE enums and models the cap attribute and
EG_LineJoinProperties choice group on CT_LineProperties.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

feature: add cap_style and join_style to LineFormat

Add _Cell.borders property providing access to per-edge border line
formatting on table cells. Each edge (left, right, top, bottom) returns
a LineFormat object for controlling border color, width, and dash style.

Uses the a:lnL, a:lnR, a:lnT, a:lnB child elements of a:tcPr which
are CT_LineProperties elements per the OOXML spec.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>