encoding/unicode: fix surrogate pair handling in UTF-16 decoder

[mohammadmseet-hue]

The isHighSurrogate function in the UTF-16 decoder incorrectly
checks the low surrogate range (U+DC00..U+DFFF) instead of the
high surrogate range (U+D800..U+DBFF).

func isHighSurrogate(r rune) bool {
    return 0xDC00 <= r && r <= 0xDFFF // BUG: low range
}

This causes two consecutive low surrogates to be incorrectly
consumed as a 4-byte surrogate pair, producing 1 replacement
character instead of 2. N consecutive low surrogates produce
N/2 replacement characters instead of N, causing data loss.

This violates:

• Unicode Standard Chapter 3, Table 3-8 (surrogate pair
  definition: high U+D800..U+DBFF then low U+DC00..U+DFFF)
• WHATWG Encoding Standard (each unpaired surrogate produces
  one U+FFFD)

Go's own unicode/utf16.Decode handles this correctly.

The fix:

• Correct isHighSurrogate to check U+D800..U+DBFF
• Add isLowSurrogate for U+DC00..U+DFFF
• Only attempt pair decoding when first code unit is a high
  surrogate and second is a low surrogate
• Unpaired surrogates of either type produce U+FFFD via the
  existing RuneLen check

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR (HEAD: 96288b8) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/text/+/763560.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
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[gopherbot]

Message from Mohammad Seet:

Patch Set 2:

(1 comment)

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