data(symbol): a prefix owns its slots only through its own row

map/symbol.src:12 says "Bare 1-9 choose from a prefix search; keep digit
aliases in matching slots", map/README:55 repeats it, and both
tests/engine_test.c:1001 and tests/mkemoji_test.py were written to it.
None of it happened.  dictprefix ends in below(), which returns a node's
own words first and then its children in rune order (dict.c:29-39), and
under `^` the punctuation sorts ahead of the digits:

	^ gave  ⁽ ⁾ ⁺ ⁻ ⁰ ¹ ² ³ ⁴ ⁵ ...
	_ gave  ₍ ₎ ₊ ₋ ₀ ₁ ₂ ₃ ₄ ₅ ...
	< gave  ← ♥ 🫰 🫶 ≤ ≠

so the file's own aliases picked the wrong character every time:

	          before   after
	^ then 1  ⁽        ¹
	^ then 2  ⁾        ²
	_ then 1  ₍        ₁
	< then 3  🫰        ♥

mkemoji has no bare `^` row to emit, because no source row claims `^` as
an alias -- the prefix exists in the trie only as the parent of `^1`..`^9`
and `^(`..`^n`, and a parent has no words of its own.  Giving the nine
superscripts `^` as a second alias, the nine subscripts `_`, and ←≤♥≠ `<`,
makes build() group them in source order and emit three rows:

	<	← ≤ ♥ ≠
	^	¹ ² ³ ⁴ ⁵ ⁶ ⁷ ⁸ ⁹
	_	₁ ₂ ₃ ₄ ₅ ₆ ₇ ₈ ₉

which is what engine_test.c:1001's fixture has said all along, and the
first thing below() now returns.  Nothing is lost: `^0`, `^(`, `^i` and
their kind still answer their own key, `0` is not a selection key so it
still extends the query, and addkouho drops the duplicate when a child
repeats what the parent already offered.  Three rows on 18953.

tests/mkemoji_test.py was asking the wrong question.  It checked that the
first nine `^` keys in the file are `^1`..`^9` -- true before and after,
and decided nothing, because the file's order is not the trie's.  It now
checks the row the engine actually reads, and fails without this change
with a bare KeyError on `^`.

What this does not cover: `+1` and `-1`.  They are not slot aliases, they
are words -- plus one, minus one -- and ➕ and ➖ are the right first
answers to `+` and `-`.  👍 is slot 3 of `+` and reachable there.

map/emoji.dict

@@ -8,6 +8,7 @@
 :(	☹
 :)	☺
 <-	←
+<	← ≤ ♥ ≠
 <=	≤
 <3	♥ 🫰 🫶
 <>	≠
@@ -16,6 +17,7 @@
 =>	⇒
 >=	≥
 ^1	¹
+^	¹ ² ³ ⁴ ⁵ ⁶ ⁷ ⁸ ⁹
 ^2	²
 ^3	³
 ^4	⁴
@@ -25,6 +27,7 @@
 ^8	⁸
 ^9	⁹
 _1	₁
+_	₁ ₂ ₃ ₄ ₅ ₆ ₇ ₈ ₉
 _2	₂
 _3	₃
 _4	₄