TP-119: "only on" layer intersection toggle (union vs intersection)

[meintsot]

Summary

When 2+ "only on" filter items are selected, the user can now choose whether an operation applies where any item is present (union — the existing default) or only where all of them overlap (intersection). The choice appears as two radio menu items — "Match any of these (union)" / "Match all of these (intersection)" — in the shared "only on" popup, so it works for both brushes and Global Operations (the Fill dialog).

Note on the ticket premise: TP-119 says "brush mode already does the intersection." That is not accurate in the code — brushes and Global Operations share the same filter path and both currently do union (AnyOfFilter) for multiple "only on" items. This PR adds the intersection option (reusing the existing AllOfFilter) rather than making one path match another.

Changes

• OnlyOnTerrainOrLayerFilter.create(dim, item, boolean intersection) — new overload; for a multi-item list returns AllOfFilter (intersection / min-strength) when true, else AnyOfFilter (union / max-strength). The 2-arg overload delegates with false.
• DefaultFilter — new 13-arg constructor carrying onlyOnIntersection (the existing 12-arg constructor is preserved and delegates with false, so CreateFilterOp and other callers are untouched), plus a field, isOnlyOnIntersection() getter, and Builder support.
• FilterPreset — persists the choice (onlyOnIntersection field + accessors + 10th captureFrom param). serialVersionUID unchanged; presets saved before this change load as false (union).
• BrushOptions — adds the two mutually-exclusive radio menu items (shown only for 2+ items, default "any"), wires the flag through getFilter()/setFilter() and into preset save/load.
• AllOfFilter — Javadoc updated to note the new "only on" intersection usage.
• Default behavior is unchanged everywhere (union); "except on" is intentionally untouched.

Test plan

• WPCore unit tests: 13 new tests pass (OnlyOnIntersectionFilterTest 6, DefaultFilterOnlyOnIntersectionTest 4, FilterPresetTest 3) — covers combiner semantics, create() type selection, DefaultFilter wiring, Builder, and a FilterPreset serialize/deserialize round-trip.
• Full multi-module build: mvn -DskipTests=true -pl WPGUI -am install → BUILD SUCCESS.
• Manual (Swing): with 2+ "only on" items, the two match-mode items appear (default "Match any"); "Match all" affects only the overlap; verified in both a brush and the Global Operations/Fill dialog; saving a "Match all" preset and reloading restores the selection; with a single item the items do not appear.

Known follow-ups (out of scope, non-blocking)

• Pre-existing: DefaultFilter.setDimension() does not propagate the dimension to the children of multi-item AnyOfFilter/AllOfFilter (affects the already-shipped multi-select union path and "except on" too). Worth a separate ticket.
• Minor UX: onlyOnIntersection is not reset if the "only on" list is shrunk back to a single item (inert — create() ignores the flag for single items).

Design: docs/superpowers/specs/2026-06-06-tp119-onlyon-layer-intersection-design.md
Plan: docs/superpowers/plans/2026-06-06-tp119-onlyon-layer-intersection.md

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