fix(dyn-input): support cartesian coords via "," (#35)

Typing \`,\` while DYN was on appended the character to the focused
field's buffer (treated as a European decimal point), so users
couldn't switch to cartesian coordinates mid-command. The dynamic
input was stuck in polar \`[Distance, Angle]\` whenever a base point
existed.

\`,\` now acts as the coordinate separator (matching the AutoCAD
convention): pressing it locks the current field's buffer, then
either advances within the existing cartesian set or reshapes a
polar configuration into cartesian:

- \`[Distance, Angle]\` + \`,\` on the first field → \`[X(buf), Y]\`
- \`[X, Y]\` + \`,\` on Y → \`[X, Y, Z]\`
- otherwise advances to the next field (Tab-equivalent)

\`sync_dyn_fields\` accepts both polar and cartesian shapes as valid
\`Point\` configurations so a mouse-move after the reshape doesn't
revert to polar. \`dyn_resolve_point\` and the overlay's live-value
formatter treat cartesian inputs as RELATIVE to the base point when
one exists — matching the DYN-on relative-coords convention from
#26. \`DynComponent::Z\` plus its label / value paths fill out the
3-D variant.

Closes #35

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/document.rs

@@ -20,6 +20,9 @@ pub(super) enum DynComponent {
     X,
     /// Absolute Y ordinate.
     Y,
+    /// Absolute Z ordinate (only visible after the user types a second
+    /// `,` separator from a cartesian X/Y configuration).
+    Z,
     /// Linear distance from the last point.
     Distance,
     /// Angle from the last point, in degrees.