Diff stage: snapshot/live collection modes, to_add + to_update outputs

compute_diff is a pure function over matches + collection items: new
games land in to_add.csv (carrying a confident version when matching
produced one); owned version-less entries with version_auto/approved
matches produce additive to_update.csv rows keyed by collid, consuming
distinct collids when photos show two editions; entries that already
carry a version are never touched — disagreements are reported in the
summary. Unseen collection entries are listed informationally. Live API
mode activates when BGG_API_TOKEN + username exist; otherwise the two
hand-pulled snapshot XMLs (real 2018 collection, 79 unique copies after
collid dedupe) are used, and they double as parsing fixtures in tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

data/collection_snapshot_expansions.xml

@@ -0,0 +1,65 @@
+<?xml version="1.0" encoding="utf-8" standalone="yes"?>
+<items totalitems="3" termsofuse="https://boardgamegeek.com/xmlapi/termsofuse" pubdate="Sat, 01 Aug 2026 17:53:03 +0000">
+<item objecttype="thing" objectid="177" subtype="boardgameexpansion" collid="53429542">
+<name sortindex="1">Advanced Civilization</name>
+<yearpublished>1991</yearpublished>
+<image>https://cf.geekdo-images.com/9ROB7NpxefRUmpR9UroQ8w__original/img/--qlhvNNj8zF9cWLlihdTg0jzmE=/0x0/filters:format(jpeg)/pic87459.jpg</image>
+<thumbnail>https://cf.geekdo-images.com/9ROB7NpxefRUmpR9UroQ8w__small/img/XDH667tVl2KjQ6szNjT7ceEmh5E=/fit-in/200x150/filters:strip_icc()/pic87459.jpg</thumbnail>
+<stats minplayers="2" maxplayers="8" minplaytime="360" maxplaytime="480" playingtime="480" numowned="3951">
+<rating value="N/A">
+<usersrated value="3412"/>
+<average value="8.00904"/>
+<bayesaverage value="6.91645"/>
+<stddev value="1.65908"/>
+<median value="0"/>
+<ranks>
+<rank type="subtype" id="1" name="boardgame" friendlyname="Board Game Rank" value="Not Ranked" bayesaverage="6.91645"/>
+<rank type="family" id="5497" name="strategygames" friendlyname="Strategy Game Rank" value="Not Ranked" bayesaverage="7.07351"/>
+</ranks>
+</rating>
+</stats>
+<status own="1" prevowned="0" fortrade="0" want="0" wanttoplay="0" wanttobuy="0" wishlist="0" preordered="0" lastmodified="2018-08-07 12:29:32"/>
+<numplays>0</numplays>
+</item>
+<item objecttype="thing" objectid="169784" subtype="boardgameexpansion" collid="53429953">
+<name sortindex="1">Castle Panic: The Dark Titan</name>
+<yearpublished>2015</yearpublished>
+<image>https://cf.geekdo-images.com/ne5GT5tNupCbGeN4maV0aw__original/img/Pe0unjKPwaO0cJPd9QI_Eo-xuRU=/0x0/filters:format(jpeg)/pic6967876.jpg</image>
+<thumbnail>https://cf.geekdo-images.com/ne5GT5tNupCbGeN4maV0aw__small/img/RTQtsNpxLidMnDPDGTTf-AiP9OQ=/fit-in/200x150/filters:strip_icc()/pic6967876.jpg</thumbnail>
+<stats minplayers="1" maxplayers="6" minplaytime="60" maxplaytime="60" playingtime="60" numowned="4511">
+<rating value="N/A">
+<usersrated value="1049"/>
+<average value="7.2278"/>
+<bayesaverage value="6.00948"/>
+<stddev value="1.15443"/>
+<median value="0"/>
+<ranks>
+<rank type="subtype" id="1" name="boardgame" friendlyname="Board Game Rank" value="Not Ranked" bayesaverage="6.00948"/>
+</ranks>
+</rating>
+</stats>
+<status own="1" prevowned="0" fortrade="0" want="0" wanttoplay="0" wanttobuy="0" wishlist="0" preordered="0" lastmodified="2018-08-07 12:44:09"/>
+<numplays>0</numplays>
+</item>
+<item objecttype="thing" objectid="104590" subtype="boardgameexpansion" collid="53430550">
+<name sortindex="1">Castle Panic: The Wizard's Tower</name>
+<yearpublished>2011</yearpublished>
+<image>https://cf.geekdo-images.com/ZI_7riAQtSAb9T3bodOVKA__original/img/S5nr9djtA4cUDvi7dmtTiYhut1Y=/0x0/filters:format(jpeg)/pic6966120.jpg</image>
+<thumbnail>https://cf.geekdo-images.com/ZI_7riAQtSAb9T3bodOVKA__small/img/VzbEp7j_wYr0KiXfvsYITTAhnSk=/fit-in/200x150/filters:strip_icc()/pic6966120.jpg</thumbnail>
+<stats minplayers="1" maxplayers="6" minplaytime="90" maxplaytime="90" playingtime="90" numowned="10158">
+<rating value="N/A">
+<usersrated value="3587"/>
+<average value="7.43829"/>
+<bayesaverage value="6.63785"/>
+<stddev value="1.18971"/>
+<median value="0"/>
+<ranks>
+<rank type="subtype" id="1" name="boardgame" friendlyname="Board Game Rank" value="Not Ranked" bayesaverage="6.63785"/>
+<rank type="family" id="5499" name="familygames" friendlyname="Family Game Rank" value="Not Ranked" bayesaverage="6.78135"/>
+</ranks>
+</rating>
+</stats>
+<status own="1" prevowned="0" fortrade="0" want="0" wanttoplay="0" wanttobuy="0" wishlist="0" preordered="0" lastmodified="2018-08-07 13:04:56"/>
+<numplays>0</numplays>
+</item>
+</items>