docs: describe trophy statistics in USAGE

docs/user/USAGE.md

@@ -159,13 +159,15 @@ Games ask for the console's language with `sceSystemServiceParamGetInt` and show
 
 ### Trophies
 
-A game unlocks a trophy by posting a `_UnlockTrophy` event with an int32 `_trophy_id` property to `libSceNpUniversalDataSystem`. The unlock is stored in `_trophy/<user id>_<service label>.txt` in the directory the game runs in (the one that holds `app0/`), one `<trophy id> <unlock time>` line per trophy; the time is an RTC tick, microseconds since 0001-01-01 UTC. Delete the file before starting the game to lock its trophies again. Each new unlock prints one line, such as `[trophy] PPSA01556 unlocked trophy 12 for user 268435456`; unlocking a trophy again prints nothing. `sceNpTrophy2GetTrophyInfo`, `sceNpTrophy2GetTrophyInfoArray` and `sceNpTrophy2GetGameInfo` report the stored unlocks and their times to the game. Every posted `_trophy_id` of 0 or more is stored, even one the trophy data has no trophy for or makes the platinum or a progressive trophy.
+A game unlocks a trophy by posting a `_UnlockTrophy` event with an int32 `_trophy_id` property to `libSceNpUniversalDataSystem`, or through the statistics rules described below. The unlock is stored in `_trophy/<user id>_<service label>.txt` in the directory the game runs in (the one that holds `app0/`), one `<trophy id> <unlock time>` line per trophy; the time is an RTC tick, microseconds since 0001-01-01 UTC. Delete the file before starting the game to lock its trophies again. Each new unlock prints one line, such as `[trophy] PPSA01556 unlocked trophy 12 for user 268435456`; unlocking a trophy again prints nothing. `sceNpTrophy2GetTrophyInfo`, `sceNpTrophy2GetTrophyInfoArray` and `sceNpTrophy2GetGameInfo` report the stored unlocks and their times to the game. Every posted `_trophy_id` of 0 or more is stored, even one the trophy data has no trophy for or makes the platinum or a progressive trophy.
 
 The trophy data comes from the game's own `app0/sce_sys/trophy2/trophyNN.ucp`, where `NN` is the service label (`00` for most games). The trophy queries report its names, descriptions and rewards in the console language (`ANYPS5_LANGUAGE`, see above), or in English (United States) or the data's default language when it has no text in that language, and its grades, groups, hidden flags, number of trophies and grade counts. A platinum trophy unlocks as soon as every trophy that names it in `platinumTrophyId` is unlocked. The first trophy query or event prints what was read, such as `[trophy] /app0/sce_sys/trophy2/trophy00.ucp: 52 trophies, 1 group, texts in en-US`.
 
-`APS5_TRACE_UDS=1` prints every posted event, such as `[uds] context 1 posted _UnlockTrophy {"_trophy_id":12}`, and a `[uds]` line for a missing `trophyNN.ucp`. An event that is not used prints the reason in a `[trophy] ignored` line: an `_UnlockTrophy` event without an int32 `_trophy_id` of 0 or more, or posted to a context that does not exist, and the first `_UpdateTrophyProgress` event of each trophy, because trophy progress is not recorded.
+Many games unlock trophies through statistics instead: `app0/sce_sys/uds/udsNN.ucp` names, for each event, the property whose value becomes a statistic, and a trophy's `unlockCondition` compares a statistic with a target. The latest value of each statistic is stored in `_trophy/<user id>_<service label>.stats.txt`, one `<stat id> <value>` line per statistic, and is kept across runs; delete it together with the unlock file to start over. A progressive trophy reports its statistic as progress. Only statistics that keep the latest value (`"aggregation":"latest"`) as `uint32` or `uint64`, and rules that copy one property of an event chosen by its name (`$.name` or `$.object.name`), are applied; the others are listed in `[trophy]` lines when the file is read.
 
-A missing or malformed `trophyNN.ucp` makes the queries report placeholders, and a malformed one prints the reason once. Icons are not read: `sceNpTrophy2GetGameIcon`, `sceNpTrophy2GetGroupIcon`, `sceNpTrophy2GetTrophyIcon` and `sceNpTrophy2GetRewardIcon` stop the game with an error. So does an unlock file in `_trophy/` that is damaged (a line that is not a new `<id> <value>` pair, such as a line an edit left blank or ended with a carriage return) or cannot be read or written. No notification is shown, and callbacks registered with `sceNpTrophy2RegisterUnlockCallback` are not called.
+`APS5_TRACE_UDS=1` prints every posted event, such as `[uds] context 1 posted _UnlockTrophy {"_trophy_id":12}`, every statistic an event sets, such as `[uds] stat 10 of user 268435456 is 10 (COLLECT $.counter)`, and a `[uds]` line for a missing `trophyNN.ucp` or `udsNN.ucp`. An event that is not used prints the reason in a `[trophy] ignored` line: an `_UnlockTrophy` event without an int32 `_trophy_id` of 0 or more, or posted to a context that does not exist; the first `_UpdateTrophyProgress` event of each trophy, because that event is not interpreted; and, once per rule, an event whose property is missing, negative, not an integer or outside the statistic's range.
+
+A missing or malformed `trophyNN.ucp` or `udsNN.ucp` turns off only what it provides, and a malformed one prints the reason once: without trophy data the queries report placeholders and statistics unlock nothing, and without statistics rules events change no statistic. Icons are not read: `sceNpTrophy2GetGameIcon`, `sceNpTrophy2GetGroupIcon`, `sceNpTrophy2GetTrophyIcon` and `sceNpTrophy2GetRewardIcon` stop the game with an error. So does an unlock or statistics file in `_trophy/` that is damaged (a line that is not a new `<id> <value>` pair, such as a line an edit left blank or ended with a carriage return) or cannot be read or written. No notification is shown, and callbacks registered with `sceNpTrophy2RegisterUnlockCallback` are not called.
 
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