feat: introduce local-first SVG workbench

docs/PATH_EDITOR.md

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+# Path editor
+
+The project-authored path core accepts every SVG path command: `M/m`, `L/l`,
+`H/h`, `V/v`, `C/c`, `S/s`, `Q/q`, `T/t`, `A/a` and `Z/z`, including repeated
+parameter groups, implicit lines after move, packed arc flags, exponents,
+relative forms and multiple subpaths. It rejects non-finite values, malformed
+arity/flags and paths beyond the command limit.
+
+Segments resolve to absolute geometry while retaining source command/form and
+fragment offsets for the command table. Serialization intentionally normalizes
+edited geometry; no editor metadata is emitted.
+
+## Controls
+
+- Every segment endpoint is an anchor.
+- Cubics show both controls. `S` reflects the previous cubic control and marks
+  the resulting handle as derived/dashed until edited.
+- Quadratics show their one control. `T` reflects the previous quadratic
+  control and marks it derived.
+- Arcs expose endpoint, radii and derived center controls and show rotation,
+  large-arc and sweep flags in the command panel.
+- Handle radii and strokes use screen-oriented overlay styling so they remain
+  usable across zoom levels.
+
+The full selected-element/ancestor transform chain maps local path geometry into
+root space. Pointer coordinates use its inverse, including an active preview
+matrix; non-invertible chains disable handles. Dragging commits one transaction
+on release; arrow keys use 1, Shift+10 or Alt+0.1 units and merge safely.
+
+Segment splitting uses the mathematical midpoint: linear interpolation for
+lines, de Casteljau for quadratic/cubic curves, and arc subdivision preserving
+the ellipse. Path reversal reverses every supported segment/subpath and adjusts
+controls/sweep where required.
+
+## Current limits
+
+The 0.1.0 UI supplies drag/nudge, split at 50%, reverse and a read-only numeric
+command table. It does not yet supply node modes, multi-selection, deletion,
+conversion, open/close, join/break/combine, subpath-start changes, arbitrary
+split parameter, pen creation, snapping, simplification or boolean operations.
+Visual editing expands shorthand/relative commands to normalized absolute data;
+the dialog/help discloses that source-form change.