Adds CodePointToString(CP: Integer): string, the inverse of CodePointAt /
CodePointFromByteIndex. It encodes a Unicode codepoint (0..U+10FFFF) as
its 1..4 byte UTF-8 sequence, providing the codepoint-aware replacement for
the Char(n) cast used in other Pascal dialects. Chr(n) only writes a single
raw byte, which is invalid for codepoints above 127; CodePointToString emits
the correct multi-byte form. Out-of-range values yield an empty string.
E2E tests cover single-byte ASCII, the two-byte 0xC2 0xB1 encoding of U+00B1,
a round-trip across all four UTF-8 length classes, and out-of-range handling.
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