fix(links): attach a link to every span its rect covers (#42)

[Anai-Guo]

Fixes #42.

Problem

merge_links maps each link annotation onto exactly one span:

max_intersection = intersection_link.argmax()
...
span_link_map.setdefault(max_intersection, [])
span_link_map[max_intersection].append(link)

A linked phrase is rarely a single span, though — spans get split whenever the font
changes and at the zero-width spacers pdfium emits between words. So for a link over
"Claude 3.7 Sonnet", only the span with the largest overlap keeps the url and every
other word of the same anchor comes back with "url": "", which is what #42 reports.

Fix

Attach the link to every span it substantially covers, normalizing the overlap by
min(span_area, link_area). That keeps both directions:

• a short span lying entirely inside the link (Sonnet) → ratio 1.0, matched;
• a long span that swallows the whole link (only one word of a sentence is linked)
  → ratio 1.0, matched, and _reconstruct_spans then splits it char by char, exactly
  as before;
• the sliver of the neighbouring line a link rect often reaches into → small ratio,
  ignored, so links don't bleed across lines.

The span with the largest overlap is always kept, so nothing that resolves today
stops resolving. _reconstruct_spans is unchanged and still decides per character,
so a partially covered span is split rather than tagged wholesale.

Verification

Reproducer — one link annotation over two runs in different fonts:

page.insert_text((left, baseline), "Claude ", fontname="helv", fontsize=18)
page.insert_text((left + w1, baseline), "Sonnet", fontname="hebo", fontsize=18)
page.insert_link({"kind": fitz.LINK_URI, "uri": url,
                  "from": fitz.Rect(left, baseline - 18, left + w1 + w2, baseline + 4)})

before:

span 'Claude ' url='https://example.com/anchor'
span 'Sonnet'  url=''

after:

span 'Claude ' url='https://example.com/anchor'
span 'Sonnet'  url='https://example.com/anchor'

test_link_spans_whole_anchor covers this and also asserts the unlinked line right
below stays unlinked. It fails on master (assert 'Claude' == 'Claude Sonnet') and
passes with the change. Full suite: 43 passed before, 44 passed after.

🤖 Generated with Claude Code