Design: editing a quote UPDATES its dimension instead of appending a rival to it

Type a new length into a sketch quote and the number changed while the geometry
sat still, with the solver dropping to "Conflicting constraints" — a broken
constraint state the user never asked for, arrived at by doing the one thing the
status line invited.

An experiment separated the two candidate causes. Same gesture, one variable:
a line committed WITH a driving length refused the edit and conflicted; a line
committed with none (Esc keeps it as drawn) accepted it and visibly shrank. So
the value was never the problem and the solver was not wrong — it was being
handed two contradicting constraints and correctly declining to choose.

The quote-edit path appended unconditionally:

    a.con = int(m_constraints.size());
    m_constraints.push_back(constraint_for(a));

Accepting the length at draw time creates Distance(P0,P1) = 64.9. Clicking the
quote later creates a SECOND Distance on the same two points asking for 30. Over
-constrained by construction. A line with no dimension yet only ever gets one
constraint, which is exactly why this looked intermittent rather than total.

upsert_constraint() finds an existing constraint with the same type and operands,
overwrites its value and returns its index; it appends only when there is none.
Operand order is ignored — a Distance from A to B is the same constraint as B to
A, and so is an Angle. Returning the index matters as much as the update: it
keeps the annotation's `con` pointing at the constraint that is actually live, so
the NEXT edit is an update too rather than reverting to appending after one good
round. upsert_dimension() applies the same rule to the visible quote, which had
been stacking labels reading different values on the same pixel, and keeps the
existing label position so a placed quote does not teleport.

set_dimension_value already did the right thing through a.con. The machinery
existed; these two call sites never consulted it.

Verified on :11 on the exact failing case — draw a line, Return to lock the
length, Confirm, double-click to re-open, click the line, type 30 into the quote:
the line shrinks, the quote reads 30.0 mm, one label not two, and the solver
stays at "3 degrees of freedom" with no conflict.

NOT included: record_dimension_constraint() has the same unconditional push_back
in all six of its branches. It belongs to the legacy Constrain mode with
different selection semantics and I could not exercise it, so it is flagged
rather than changed blind.

Refs snaporca-e1p.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LyRwbuq6fjn3VV9U9UvhBM

src/slic3r/GUI/DesignSketchTool.hpp

@@ -620,6 +620,10 @@ private:
     double measure_dim(const DimAnnot& a) const;                            // value from geometry
     std::string dimtype_title(DimType k) const;
     SketchEntityConstraintDef constraint_for(const DimAnnot& a) const;      // driving def
+    // One driving constraint (and one visible quote) per kind+operands: re-typing a value must
+    // UPDATE it, not append a rival asking for something else. Both return the index.
+    int upsert_constraint(const SketchEntityConstraintDef& c);
+    int upsert_dimension(const DimAnnot& a);
     int  place_dimension(DimAnnot a);                                       // create+drive+notify
     std::string dim_text(const DimAnnot& a) const;                          // rendered label string
     void render_dimensions(double unit_per_px);                            // quote lines + labels