perf: batch table string insertions

[vincentarelbundock]

Summary

Batch generated Typst and HTML content before calling lines_insert(), reducing repeated splitting and collapsing of the growing table string without introducing a parallel line-oriented helper API.

This is a smaller alternative to #666. It keeps the existing lines_insert() abstraction and changes only the hot call sites:

• batch Typst horizontal and vertical rules
• batch Typst style-array entries and notes
• batch HTML span listeners, style arrays, and CSS entries
• flatten nested vertical-line results before insertion
• preserve the existing insertion order byte-for-byte

Benchmarks

Median build_tt() rendering time, measured in separate main and simpler666 worktrees after warm-up. Primary cases used 30 iterations.

Workload	main	This PR	Change
Typst, plain 20x8	9.54 ms	9.30 ms	2.5% faster
Typst, 320 distinct styles	88.60 ms	35.61 ms	59.8% faster
Typst, 200 notes	35.11 ms	13.27 ms	62.2% faster
HTML, plain 20x8	17.31 ms	17.37 ms	unchanged
HTML, 320 distinct styles	569.16 ms	107.23 ms	81.2% faster
Typst, many borders on 500x10	3815 ms	3088 ms	19.1% faster
HTML, many borders on 500x10	3821 ms	3833 ms	unchanged

Verification

• Focused Typst tests: 32 passed
• Full scrutin suite: 437 passed, 0 failed, 1 skipped
• git diff --check passes

The issue #592 snapshot is covered and remains byte-for-byte unchanged.