docs: describe trophy unlocks in USAGE

docs/user/USAGE.md

@@ -159,9 +159,13 @@ Games ask for the console's language with `sceSystemServiceParamGetInt` and show
 
 ### Trophies
 
-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. The first trophy query prints what was read, such as `[trophy] /app0/sce_sys/trophy2/trophy00.ucp: 52 trophies, 1 group, texts in en-US`.
+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.
 
-Trophies are not unlocked yet: every trophy reports as locked, and nothing is written for them. A missing or malformed `trophyNN.ucp` makes the queries report placeholders, and a malformed one prints the reason once; `APS5_TRACE_UDS=1` prints a `[uds]` line for a missing one. Icons are not read: `sceNpTrophy2GetGameIcon`, `sceNpTrophy2GetGroupIcon`, `sceNpTrophy2GetTrophyIcon` and `sceNpTrophy2GetRewardIcon` stop the game with an error. No notification is shown, and callbacks registered with `sceNpTrophy2RegisterUnlockCallback` are not called.
+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.
+
+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.
 
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