extradoc: word-atomic in-paragraph diff for readable redlines

[cassus]

Context & disclosure. This is a readability enhancement (not a bug fix)
that I've used and verified on real documents in SUGGEST mode. The change, the
tests, and this write-up are largely AI-generated (Claude Opus); the prose and
code are largely AI-authored, so please weight the detail accordingly.

It's motivated by SUGGEST mode, which isn't generally available yet and
whose main PR hasn't landed — so it's completely fine if you'd rather not merge
this now. I'm opening it mainly to share the approach upstream. Note it also
changes the diff granularity for direct edits, which I haven't specifically
exercised — see the trade-off below.

This project has been really useful to me and I'd like to give back: if you want
more human attention, deeper testing, or changes to make this mergeable, I'm
happy to engage personally and put in the work — just say the word.

What

Diff paragraph bodies at word-token granularity instead of character level,
so multi-word edits stay single readable chunks in the Docs Suggestions UI
rather than fragmenting into char-level "insert/delete soup". A char-level
fallback kicks in for whitespace-only edits (e.g. "PARTI" -> "PART I") so a
single inserted/removed space stays a minimal edit rather than replacing the
whole token.

Trade-off / scope

This changes the opcodes produced by _diff_paragraph_runs, which affects any
consumer of paragraph-level diffs, not just SUGGEST-mode redlines. The intent
is purely cosmetic (fewer, more meaningful suggestion chunks); it does not
change which characters end up in the final document. Happy to gate it behind a
flag if you'd prefer to keep char-level diffing as the default.

Changes

• reconcile_v3/lower.py — _word_diff_opcodes / _tokenize_words; wire
  _diff_paragraph_runs to use them.
• tests/reconcile_v3/test_lower.py — TestWordAtomicDiff: a one-word change
  stays one atomic replace opcode (plain char-level difflib fragments
  "brown"->"red" on a spurious shared 'r'); a whitespace-only change
  falls back to a minimal char-level insert.

Verification

• Full unit suite green (703 passed locally).