unicode/norm: do not compose across a ccc=0 character that combines backward

[tannevaled]

UAX #15, D115: a character C is blocked from starter A when some B between
them has ccc(B) = 0 or ccc(B) >= ccc(C). reorderBuffer.compose tracks the
current starter in s, but only ever updates it inside the combinesBackward
branch. That is enough for a ccc = 0 character that does not combine
backward, because such a character ends the segment and never shares a
buffer with an earlier starter. One that does combine backward has to stay
in the buffer, since it may compose with what precedes it; when it does not,
it falls through with s still pointing at the earlier starter, and the next
mark composes across it.

norm.NFC.String("iা̖̀")
  got  "ìা̖"       -- i + U+0300 composed across U+09BE
  want "iা̖̀"

Set s when the character just written is itself a starter.

The bug is not new and not tied to a particular Unicode version. Of the 35
characters in Unicode 17 that have ccc = 0 and are the second element of a
canonical composition, NFC(<i, S, U+0300, U+0316>) fails to keep the "i" for
24 of them under the Unicode 15 tables and 33 under Unicode 17 -- among them
U+09BE Bengali, U+0BBE Tamil, U+0D3E Malayalam, U+1B35 Balinese and U+102E
Myanmar. The nine that differ between the two table sets are simply those
Unicode 15 had not assigned yet. The two that never fail, U+0FB5 and U+0FB7,
are composition exclusions.

The added test uses U+09BE, which is assigned with ccc = 0 in both table
sets, so it does not depend on which toolchain selects which tables, and it
is paired with U+0903 -- ccc = 0, does not combine backward -- so the two
cases sit either side of the distinction.

Verified beyond the package's own tests: NFC, NFKC and NFD of 3112063 inputs
were compared before and after, and every one of the 2778 (Unicode 15) and
4153 (Unicode 17) differing results was judged by a separate UAX #15
implementation built from UnicodeData.txt, itself checked against
NormalizationTest.txt at 305184 and 320544 assertions with no failures. All
of the differences are corrections; there are no regressions.

Fixes golang/go#81001

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