Remember custom personality graphs and correct default resets

docs/personality_custom_graphs.md

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+# Custom personality graphs
+
+Each personality keeps its own Custom acceleration, braking and following curve.
+Selecting a named preset changes the active selection without deleting Custom
+points. Selecting Custom again restores those points, including after a reload
+or restart. If a category has never had Custom points, it is initialized from
+the current selection, as before.
+
+The existing **Reset to default** button, below each Custom graph's numeric
+points in New Galaxy's Advanced section, replaces only that category's Custom
+curve. It leaves the category set to Custom. The server resolves the reset
+values; the dashed **Dom default** line uses the same resolver.
+
+Defaults are Dom's configured base curves sampled at the editor's 10 mph
+points. They include Traffic's dedicated acceleration and braking, following
+settings, global tuning switches and powertrain overrides. Where gear mapping
+is enabled, the reference uses normal gear. Live Eco/Sport gear, weather,
+lead/stop and overspeed adjustments remain on the existing controller paths.
+Sampling cannot reproduce every native breakpoint or between-point value;
+resetting a Custom graph is not the same as delegating to the Dom-default
+runtime path.
+
+Dom-default points outside the ordinary editor range (such as Traffic braking
+at 0.35 m/s², configured Traffic following at 0.5 seconds or truck acceleration
+at 6 m/s²) remain visible and are preserved when another point is edited.
+New point edits still use the existing authoring bounds. This does not expand
+braking authority or change named-preset controller behaviour.
+
+## Storage compatibility
+
+Profile document version 3 retains `curve` and optional `legacyCurve` while
+`preset` is a named preset or `dom_default`. These retained values are dormant;
+only Custom uses them. An actual graph edit or reset retires preserved v1
+interpolation for that category; a preset switch or unchanged submission does
+not.
+
+Valid v2 documents are read losslessly and upgraded on the next normal write.
+Version 1 keeps its existing explicit, verified migration flow. Reads never
+rewrite Params. Category conflict detection, off-road checks and atomic profile
+document writes still apply to edits and resets.
+
+Older builds do not understand v3 documents. Retain a compatible settings
+backup before rolling back to one of those builds. Curves discarded before
+this change cannot be recovered automatically.