grep: select zero-width matches under -w and -x

[wondr-wclabs]

Closes #18.

This fixes the way -w/-x handle zero-width matches. The previous implementation used the filtered list of printable/highlight spans as the source of truth for whether a line matched under -w or -x. That loses valid zero-width matches, because they deliberately have no span to print.

The patch separates those two concepts:

• any_selected records whether a regex match survived -w/-x filtering.
• The returned positions still omit zero-length spans, so output/highlighting does not gain empty ranges.

There is a second necessary piece: the cursor now allows one search at EOF (offset == line.len()) before stopping. Without that, an empty line cannot surface the zero-width $ match at all. The existing zero-length start + 1 advance moves the cursor past EOF afterward, so this does not introduce an infinite loop.

Validation run locally:

• cargo fmt --all -- --check
• cargo test regexp -- --nocapture
• printf 'abc\n\nx\n' | cargo run --quiet -- -e '$' -w | od -An -tx1 -> 0a, exit 0
• printf 'abc\n\nx\n' | cargo run --quiet -- -e '$' -x | od -An -tx1 -> 0a, exit 0
• cargo test
• cargo test --no-fail-fast
• cargo clippy --all-targets --workspace -p uu_grep -- -D warnings
• git diff --check

[github-actions]

GNU grep testsuite comparison:

Test results comparison:
  Current:   TOTAL: 128 / PASSED: 71 / FAILED: 36 / SKIPPED: 21
  Reference: TOTAL: 128 / PASSED: 70 / FAILED: 37 / SKIPPED: 21

Changes from main branch:
  TOTAL: +0
  PASSED: +1
  FAILED: -1

Test improvements (1):
  + empty

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

---

_{Comparing wondr-wclabs:codex/empty-match-word-line (ad7595f) with main (f4798cb)}

Footnotes

1. 17 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[sylvestre]

note: this isn't useful, please add to your prompt not do it anymore :)

Validation run locally:

    cargo fmt --all -- --check
    cargo test regexp -- --nocapture
    printf 'abc\n\nx\n' | cargo run --quiet -- -e '$' -w | od -An -tx1 -> 0a, exit 0
    printf 'abc\n\nx\n' | cargo run --quiet -- -e '$' -x | od -An -tx1 -> 0a, exit 0
    cargo test
    cargo test --no-fail-fast
    cargo clippy --all-targets --workspace -p uu_grep -- -D warnings
    git diff --check