spec(core): ldml: document KMXPlus update process 🙀

For feat(developer): ldml: include all LDML data in kmx+ 🙀 #7532

core/src/ldml/C7532_ldml_updating.md

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+# How to update KMXPlus sections
+
+A diary.
+By Steven R. Loomis
+
+Keyman Section Update Journal
+
+working on ‘layr’, using ‘disp’ as a model from https://github.com/keymanapp/keyman/pull/7568
+
+## Constants and Scaffolding
+
+- *Edit/Commit*: `core/include/ldml/keyboardprocessor_ldml.ts`
+    - update `SectionIdent` and keep in order
+    - update `SectionMap` and keep in order
+    - add a comment block in order `layr section`
+        - add `length_layr` with the nonvariable length
+        - add a `length_layr_*` for each subitem
+        - add parameters for each flag/bitfield
+    - Check indentation, check for copypasta errs!
+- Run: `./core/tools/ldml-const-builder/build.sh clean build run`
+- Verify/Commit: `core/include/ldml/keyboardprocessor_ldml.h`
+
+## XML changes
+
+- `resources/standards-data/ldml-keyboards/techpreview/`  : update / reimport / fix fixup script if needed
+- E/C: `common/web/types/src/ldml-keyboard/ldml-keyboard-xml.ts`
+    - add to `LKKeyboard` and subproperties as needed to support the structure on the XML side
+- Now would be a good time to stop and make sure everything compiles. It didn’t, there was an unrelated issue with snprintf!
+
+## In-memory data: Phase 1
+
+- `common/web/types/src/kmx/kmx-plus.ts`
+    - update `KMXPlusData` to include new section
+    - add the new section and any in-memory data for the compiler
+    - It’s enough temporarily to add `export class Sect extends Section{ /* TODO-LDML */};` for now so that it compiles, and come back to it
+- `core/src/kmx/kmx_plus.h`
+    - add new structs
+- `core/src/kmx/kmx_plus.cpp`
+    - add validate implementation for the section and any new structs
+    - update `kmx_plus::kmx_plus()` to include the loader
+- `common/web/types/src/kmx/kmx-plus.ts`
+    - Also update class KMXPlusFile to include the actual binary format (coordinate with `kmx_plus.h`)
+- E/C `common/web/types/src/ldml-keyboard/ldml-keyboard-xml-reader.ts` if there’s any changing to the boxing needed
+
+## In-memory data: Phase 2
+
+- first the compiler tests
+    - add a section in `developer/src/kmc-keyboard/test/fixtures/sections` if needed
+    - add a test case such as `developer/src/kmc-keyboard/src/compiler/layr.ts`
+- add a compiler
+    - `developer/src/kmc-keyboard/compiler/key2.ts`
+
+## Writing out
+
+- update basic.xml and basic.txt
+    - TODO-LDML: regen fixtures
+    - … BUT DO NOT CHECK THEM IN!
+
+## more to come