Add Paragraph#substitute for placeholders split across runs (#147)

[satoryu]

Summary

Fixes #147. Adds Paragraph#substitute(pattern, replacement), which can replace a placeholder even when Word has split it across multiple text runs.

The bug

Word frequently splits a placeholder such as {{first_name}} across several <w:r> runs (e.g. {{fi, rst_na, me}}). The existing TextRun#substitute works on a single run, so it can never match a placeholder that spans runs:

doc.paragraphs.each { |p| p.each_text_run { |tr| tr.substitute('{{first_name}}', 'World') } }
# => no change; the placeholder is split across runs

Replacing at paragraph level via paragraph.text = ... "works" but collapses the whole paragraph into one run, losing all formatting (noted by the reporter).

The fix

Paragraph#substitute(pattern, replacement):

1. Joins the run texts into the full paragraph string.
2. Finds matches across run boundaries.
3. Collapses each match into the first run it touches (keeping that run's formatting) and empties the other runs the match spanned. Runs outside the match are untouched, so unrelated formatting is preserved.

Supports both string and regex patterns, including capture-group backreferences in the replacement (uses String#sub semantics).

doc.paragraphs.each { |p| p.substitute('{{first_name}}', 'World') }

Tests

• New fixture split_placeholder.docx — paragraph "Hello {{first_name}}!" with the placeholder split across runs ["Hello ", "{{fi", "rst_na", "me}}", "!"].
• Specs cover: the fixture really is split; string substitution preserving surrounding text; regex with a capture group; and a save/reopen round-trip.
• Confirmed the split-run case is unfixable via the old per-run API and now passes. Full suite green locally (156 examples, 0 failures).

Notes / limitations

• The match region adopts the first spanned run's formatting (a placeholder is meant to be replaced wholesale, so this is the desired behavior).
• The existing TextRun#substitute / #substitute_with_block are unchanged.

Credit to @manovasanth1227 for the clear report and the run-healing approach that informed this implementation.

Closes #147

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