Reject invalid UTF-16 surrogate pairs and lone low surrogates for clarity

The string unescape logic treated any code unit in the surrogate range

(0xD800-0xDFFF) as the start of a surrogate pair. The concrete example

from issue #39 (\uDC00\uDC00) was already rejected by the existing

0x10FFFF bounds check, but the check was misleading and would accept a

lone low surrogate as a regular BMP code point if the second surrogate

happened to be in a narrower range. Clean this up so the intent is

obvious.

Changes in src/parser3.h:

- Fast path (n_string2): only treat high surrogates (0xD800-0xDBFF) as

the start of a surrogate pair, and explicitly reject lone low surrogates

(0xDC00-0xDFFF).

- Slow path (t_hex2): same high-surrogate check, with explicit rejection

of lone low surrogates.

The t_hex3 path already validates that the second code unit is a low

surrogate (0xDC00-0xDFFF), so no change is needed there.

Also commit the current fuzzer corpus.

Closes #39

corpus/bf4a60df3e13fe1de286aaeab5e96085e6634fea

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