fix: keep marker keys unique when repeated across worlds

[Yusufihsangorgel]

Closes #2229.

The problem

When the map is zoomed out far enough that the world repeats horizontally,
MarkerLayer draws each marker once per visible world. getPositioned(worldShift)
builds a Positioned for every world and passed key: m.key unchanged, so a
marker that has a key produced several sibling Positioned widgets with the
same key inside the layer's Stack:

Duplicate keys found.
Stack(...) has multiple children with key [<'m-1'>].

Markers without a key were never affected. Only keyed markers, and only once
they became visible in more than one world.

The fix

Keep the caller's key on the main world (worldShift == 0), so find.byKey and
per-marker widget identity are unchanged, and derive a distinct key from the
world shift for the repeated copies:

key: m.key == null || worldShift == 0
    ? m.key
    : ValueKey((m.key, worldShift)),

Tests

Added test/layer/marker_key_worlds_test.dart: a wide viewport at low zoom
repeats the world, the keyed marker is drawn more than once, no duplicate-key
exception is thrown, and find.byKey still resolves to exactly one widget. I
checked it both ways: the test fails without the fix and passes with it. Full
suite green (118 tests).

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I used AI tooling to help investigate and draft this. I have reviewed the
change, wrote the test, and take full responsibility for it.