fix(matcher): include last start in unicode substring and leading-digit ignore-case

[SebTardif]

Summary

Include the last valid start when searching Unicode (and CRLF) haystacks, and search the full last-valid-start window for ignore-case ASCII needles whose first letter is not at index 0.

Problem

Two substring false negatives on current master.

Unicode and CRLF suffix miss

substring_match_non_ascii iterated haystack[start..haystack.len() - needle.len()], so the last valid start (len - needle.len()) was never tried. ASCII haystacks were fine. Any Unicode haystack, or a \r\n that forces the Unicode Utf32Str variant, missed a multi-character substring sitting at the end.

Matcher::substring_match("üfoo", "foo")  // None
Matcher::postfix_match("üfoo", "foo")    // Some(...)
Pattern::parse("'bar").match_list(["foo\r\nbar", "foo\nbar", "bar"])
// missed the CRLF row

This is leftover from #34. The exclusive end was introduced in 3d46b625 while closing that issue (panic / false positives). The bound is one short of the last valid start. The ASCII prefilter already uses haystack.len() - needle.len() + 1.

Leading-digit ignore-case window

When ignore_case is set and the first a-z in the needle is at index len >= 2, the code memmems the full needle in a window of haystack.len() - needle.len() + len (too short) and returns None with no fallback. Some(0) and Some(1) already use the correct window + 1.

This is not #100. The needles are already lowercase. Pattern / Atom fold mixed-case input before calling Matcher. The miss happens for '12foo vs both xx12FOOyy and xx12fooyy.

Matcher::substring_match("xx12FOOyy", "12foo")  // None (needle already lowercase)
Pattern::parse("'12foo").match_list(["xx12FOOyy", "xx12fooyy"])  // []

Change

In matcher/src/exact.rs:

• Unicode exclusive end is now haystack.len() - needle.len() + 1 (last valid start is included).
• Some(len) for len >= 1 uses the same path as Some(1): search the first non-letter byte on haystack[..haystack.len() - needle.len() + 1], then verify the tail with normalize.

Validation

Red (production from origin/master, new tests only):

• tests::test_substring_unicode_suffix: "foo" did not match "üfoo" (None vs Some(56))
• tests::test_substring_crlf_suffix: CRLF suffix should match: ["foo\nbar", "bar"]
• tests::test_substring_leading_digits_ignore_case: "12foo" did not match "xx12FOOyy" (None vs Some(110))

Green after the two window fixes:

• cargo test --workspace: 28 nucleo-matcher lib tests, 8 nucleo tests, and 9 doc-tests passed
• cargo fmt --all --check passed

Clippy -D warnings is already red on master (#98); not used as a gate.

Refs

Ref #34