feat(odf): read the enhanced geometry languages, formulas and path alike

[andiwand]

🤖 Generated with Claude Code

Stage 3 of #771, on main now that #797 has landed.

draw:enhanced-geometry is what makes a custom shape an arrow, a callout or a
star, and it says so in two small languages: draw:formula (20.36) and
draw:enhanced-path (19.145). Neither was read, so all 350 of them in the test
corpus rendered as rectangles.

odf_enhanced_geometry.cpp implements both as pure functions over strings.
Nothing is wired into the element model here — that is the next PR, which
keeps this one reviewable on its own and changes no output.

• evaluate_formula is recursive descent over 20.36: $N modifiers, ?name
  equation references resolved through a caller-supplied lookup, the named
  view-box values, and abs sqrt sin cos tan atan min max atan2 if. The
  trigonometric functions take radians; the corpus settles it by writing
  sin(105*(pi/180)).
• convert_enhanced_path turns every command of 19.145 into an svg d.

Both readers run off odf_scanner.hpp, which is odf_geometry.cpp's scanner
promoted to a header now that it has a second user: a std::string_view cursor
whose reads are bounded by what remains, so no reader here holds a buffer or a
const char *. A comma is filler in a coordinate list and an argument
separator in a formula, so consume skips only spaces.

Two things worth knowing about the path conversion

• An arc is emitted in segments of at most a half turn, which means the
  large-arc flag is never needed and U 10800 10800 10800 10800 0 360 — a full
  turn, which a single svg A cannot express, and 116 of the corpus's commands
  — still draws.
• F and S are read and dropped: painting one subpath with a different fill
  or stroke is more than one d can say.

Verification

odf_enhanced_geometry_test.cpp, from string literals: the geometry needs no
fixture to exercise. No rendered output changes, so no reference-output work
and no CHANGELOG.md entry — the next PR carries both.