Cache element last child during HTML append

[kitsuyui]

Summary

• Update src/lib.rs to cache each element's last child inside DocHtmlSink so text coalescing does not rescan all children on every append.
• Update or invalidate the cache when append, remove, append-before-sibling, and reparent operations can change an element's last child.
• Add a src/lib.rs flat sibling regression test that parses hundreds of span and trailing text sibling pairs.

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo check
• cargo test --all-targets --all-features
• cargo test --all-features
• cargo build --release --all-features
• scripts/test-check-cargo-version-tag.sh

Not run locally:

• cargo llvm-cov --lcov --output-path coverage.lcov (coverage reporting is left to CI)
• reusable spellcheck workflow (no documentation files changed)

[github-actions]

Code Metrics Report

	main (928e2f2)	#121 (e670b1b)	+/-
Coverage	68.3%	69.0%	+0.7%
Code to Test Ratio	1:0.1	1:0.1	-0.1
Test Execution Time	5s	11s	+6s

Details

  |                     | main (928e2f2) | #121 (e670b1b) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          68.3% |          69.0% | +0.7% |
  |   Files             |              4 |              4 |     0 |
  |   Lines             |            550 |            605 |   +55 |
+ |   Covered           |            376 |            418 |   +42 |
- | Code to Test Ratio  |          1:0.1 |          1:0.1 |  -0.1 |
  |   Code              |            593 |            652 |   +59 |
  |   Test              |             71 |             71 |     0 |
- | Test Execution Time |             5s |            11s |   +6s |

Code coverage of files in pull request scope (68.8% → 69.5%)

Files	Coverage	+/-	Status
src/handle.rs	17.4%	+6.3%	affected
src/lib.rs	76.2%	-0.9%	modified

---

Reported by octocov