fix(grep): document="EX-10.1" selects exactly EX-10.1; literal matches keep source offsets

[dgunning]

Beads edgartools-5qqy and edgartools-6th2 (salvaged from closed PR #1380).

5qqy: document="EX-10.1" also searched EX-10.10 through EX-10.19

Filing._attachment_matches matched the document type by substring. It's replaced by _select_attachments, which looks at the whole attachment list and applies a precedence: an exact filename wins, then an exact document type, and the substring match applies only when nothing matched exactly. That means document="EX-10" still selects every EX-10 exhibit. PR #1380 only added an exact-filename check ahead of the substring test, which left the type collision in place.

Ground truth, SeeQC's S-1 (0001213900-26-073222): "agreement" in EX-10.1 (a Stockholder Support Agreement) gives 133 matches. The old filter returned 378, adding the EX-10.10/11/12 employment agreements.

6th2: literal grep shifted matches after characters whose lowercase is longer

The literal branch found positions in text.lower() and sliced the original text with them. "İ".lower() is two characters, so each one before a match shifted the result by one. Literal search now runs re.escape(pattern) with IGNORECASE over the original text.

It keeps the old scan's overlapping matches ("aa" in "aaa" gives two) by searching again from m.start() + 1 rather than using finditer, which would have silently dropped them. Regex mode is unchanged.

Verification

• New fast tests: document selection (exact type, exact filename, substring fallback, primary, no filter, end-to-end grep()) and literal offsets (the İ case, source casing, overlap, regex metacharacters in a literal).
• Network test: the SeeQC S-1 case, pinned by accession.
• Against main: the offset test fails on the old grep.py; the end-to-end and SeeQC tests fail on the old _filings.py.
• Existing tests: the grep tests and the Filing search/grep don't work on text (non-HTML) filings #819 regression tests pass (129). test-fast: the only failure is the dt1f1 20-F test, which also fails on main.
• Docs: the grep() docstring and docs/guides/full-text-search.md describe the precedence.

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