docs(libSceIme): document null argument return code uncertainty

docs/dev/debt/libSceIme.md

@@ -11,6 +11,7 @@
 - [sceImeGetPanelPositionAndForm](../../../core/libs/prx/libSceIme/Export.cpp) (libSceIme) - the output is taken to be the `PositionAndForm` that `sceImeDialogGetPanelPositionAndForm` fills. While a panel is open it reports the type, position and alignments passed to `sceImeOpen` and the size `sceImeGetPanelSize` gives; the console's values for a panel moved by its own position adjustment are unknown.
 - [sceImeFilterText](../../../core/libs/prx/libSceIme/Export.cpp) (libSceIme) - the behaviour follows the disassembly of `libSceIme.sprx` from firmware 13.60 and is differential-tested against that function in an emulator, not measured on a console. [ImeFilterCodepages.hpp](../../../core/libs/prx/libSceIme/include/ImeFilterCodepages.hpp) holds the two codepage checks (helpers at 0x4850 and 0x48f0), rebuilt from Unicode's public [CP932](https://unicode.org/Public/MAPPINGS/VENDORS/MICSFT/WINDOWS/CP932.TXT) and [CP936](https://unicode.org/Public/MAPPINGS/VENDORS/MICSFT/WINDOWS/CP936.TXT) tables plus the Latin-1, CJK Extension A and a few other units the firmware also accepts (listed in the PR). The parameter layout beyond `type` is inferred: `mask` is the allowed-class bitmask, and `version` and `variant` derive it when it is 0. [sceImeCheckFilterText](../../../core/libs/prx/libSceIme/Export.cpp) throws, as its contract is unknown
 - [sceImeOpen, sceImeUpdate, sceImeSetText, sceImeSetCaret, sceImeClose](../../../core/libs/prx/libSceIme/Export.cpp) (libSceIme) - the event ids (Open 0, UpdateText 1, UpdateCaret 2, PressClose 4, PressEnter 5), the caret directions (left 1, right 2), the error codes and the text event payload follow the PS4 reimplementation in [shadPS4](https://github.com/shadps4-emu/shadPS4/blob/b72489d8c24e63fc56a29fe2c90120ab768a210d/src/core/libraries/ime/ime.cpp); none is measured on a PS5. Events are queued and delivered by `sceImeUpdate` only when it is passed the handler given to `sceImeOpen`, with that call's `arg`. A text event points `str` at a copy of the text kept in `work`, and `inputTextBuffer` holds the same text; both are terminated, `caret_index` is the caret after the edit and the one edit area has the edit position and the change in UTF-16 units (0 for Enter and Close). An edit sends one UpdateText; a caret move sends one UpdateCaret per UTF-16 unit, as shadPS4 does. Enter inserts a line break with the multiline option and sends PressEnter otherwise; Escape sends PressClose. Typed text is cut to `maxTextLength` without splitting a surrogate pair, line breaks are dropped without the multiline option, and the BasicLatin, URL, mail and number types do not filter characters. `sceImeOpen` checks, in shadPS4's order, the user, type, enter label, input method, option bits (the `sceImeGetPanelSize` mask), `maxTextLength` (1 to 2048), the text buffer, the position (below 1920x1080, or 3840x2160 with the over-2K option), the alignments, the extended priority and reserved bytes, `work` (non-null, 4-byte aligned), the handler, the reserved bytes and the initial text (no unpaired surrogate, no line break without the multiline option); shadPS4's SDK-dependent option, language and extended-option masks and its work-overlap check are not applied, and only the text copy is written to `work`. `sceImeSetText` keeps the caret unless the text became shorter, `sceImeSetCaret` rejects an index past the text with `0x80bc0030`, and neither sends an event. `sceImeSetTextGeometry` accepts any geometry while a panel is open, since no panel is drawn
+- [sceImeUpdate and sceImeKeyboardOpen](../../../core/libs/prx/libSceIme/Export.cpp) (libSceIme) - returning 0x80BC0031 (ErrorInvalidAddress) for a null parameter in `sceImeKeyboardOpen` follows the reimplementations in [shadPS4](https://github.com/shadps4-emu/shadPS4/blob/b72489d8c24e63fc56a29fe2c90120ab768a210d/src/core/libraries/ime/ime.cpp#L557-L560) and [KytyPS5](https://github.com/KytyPS5/KytyPS5/blob/1211958b4eb38256cc0c5c7ba92d668820c05389/src/libs/ime.cpp#L540-L542). In contrast, returning 0x80BC0022 (ErrorInvalidHandler) from `sceImeUpdate(nullptr)` is an assumption based on the error code name and sibling parameter checks; neither shadPS4 nor KytyPS5 returns INVALID_HANDLER for a null handler (both return NOT_OPENED when no session or keyboard is open or when no active handler matches), so the existing shadPS4 reference does not establish this result. The regression tests establish the host implementation's behavior rather than verified native return codes.
 - [sceImeForTestFunction](../../../core/libs/prx/libSceIme/Export.cpp) returns a constant error without consuming arguments in the supplied module; unused argument types cannot be recovered from that body.
 - [sceImeConfigSet and sceImeConfigGet](../../../core/libs/prx/libSceIme/Export.cpp) preserve the native argument slots: an unused first slot, a 64-bit configuration value, a 32-bit key and, for set, a 32-bit value. The service receives a 24-byte request; configuration/key semantics and unused argument types remain unresolved. The getter advertises one output transfer but does not initialize its descriptor in this module. Protected bodies remain unchanged.