Give last inline content to final text node.

[kivikakk]

Now that the inline processor is given ownership of the accumulated string content, we can use that same allocation for the last text node, instead of having to copy it.

[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./bench.sh ./comrak-65c862b	106.5 ± 0.9	105.2	109.1	1.80 ± 0.03
./bench.sh ./comrak-main	108.9 ± 1.2	106.3	110.8	1.84 ± 0.03
./bench.sh ./pulldown-cmark	59.2 ± 0.9	58.2	62.1	1.00
./bench.sh ./cmark-gfm	92.3 ± 5.8	83.9	104.1	1.56 ± 0.10
./bench.sh ./markdown-it	264.5 ± 2.9	259.0	271.3	4.46 ± 0.08

Run on Tue Oct 21 08:57:21 UTC 2025