fix: match inflected Polish in the fallback full-text search

[Irahan2]

Closes #59.

The entity_search index doesn't stem, so an oblique-case question never reached a nominative
title. build_lucene_query now also emits prefix and edit-distance clauses for tokens of 5
characters or more. For "Co się dzieje w semestrze zimowym?":

"semestrze zimowym"^2     exact phrase, unchanged
semestr*^0.5              prefix, catches "Semestr"
zimow*^0.5                prefix, catches "zimowy"
semestrze~1^0.3           edit distance, for endings the prefix trim misses
zimowym~1^0.3

The prefix trims 2 characters with a floor of 4, since Polish endings are mostly 1-3 chars. Short
tokens stay exact, a 3-char prefix matches half the graph. Both expansion boosts are below 1 so an
exact hit still outranks them.

Measured on a 33-node graph

• all 5 oblique-case questions recover their node at rank 1
• all 5 unrelated questions still return 0 rows, no precision regression
• "Kiedy zaczyna się semestr letni?" returns Semestr letni, not Semestr zimowy
• nominative query scores 10.306, the oblique form of the same question 1.384, so boost ordering
  holds

Also re-ran the #57 abstention checks against Neo4j 5.18 and they still pass.

One thing from the issue notes I'd push back on

The notes suggest tuning fallback_min_score up, since real hits were around 10 against a floor
of 0.5. After the expansion that isn't safe any more. A legitimate oblique-case hit scores about
1.4, because inflected matches are low-scoring by construction. A floor picked from nominative
scores would drop exactly the matches this change exists to recover.

So I left it at 0.5 and wrote the reason into graph_config.yaml and CLAUDE.md, so it doesn't get
raised later on the strength of the original note.

If a Polish analyzer ever ships in the deployed Neo4j build, configuring it at index creation is
the better fix and this expansion can go. That's noted in CLAUDE.md too.

Notes

• Two existing tests compared the whole Lucene string, so they now assert the quoted clauses
  instead. Same coverage, just not brittle to the extra clauses.
• Only the fallback path changes. Model-generated primary queries are untouched.

[Dominik-Galus]

lgtm