Add BIP-85 applications to Flex and Stax

[aido]

This PR adds the following BIP-85 applications for Flex and Stax devices:

• BIP-39
• Base64 passwords
• Base85 passwords

The code has also been written for other BIP-85 applications such as hex output and dice throws but a UI has not yet been added for these while I decide if they will be useful or just add bloat to the app. They may appear in a future version.
I am also debating what BIP-85 applications (if any) should be added to Nano devices.

Note

The app support page at https://support.ledger.com/article/Seed-tool may have to be updated to include BIP85 if/when this PR is merged.

This PR is currently a work in progress while I add some function tests.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.98%. Comparing base (8ab6d37) to head (f507dd7).
⚠️ Report is 7 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #14      +/-   ##
===========================================
+ Coverage    60.04%   60.98%   +0.93%     
===========================================
  Files           14       18       +4     
  Lines         1632     1671      +39     
  Branches       266      270       +4     
===========================================
+ Hits           980     1019      +39     
  Misses         612      612              
  Partials        40       40              

Flag	Coverage Δ
unittests	60.98% <100.00%> (+0.93%)	⬆️

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[aido]

Hi @lpascal-ledger,

Just a gentle reminder that this PR is ready for review whenever your team gets a chance.

Apart from bug fixes this version (v1.9,0) will probably not have any more updates with additional features and tools. For now the app is feature complete.
If there is user demand for more features (e.g. BIP-85 DICE app or BIP-85 HEX app) then those features can be considered in future versions if they are deemed to be useful and don't cause bloat.

[lpascal-ledger]

Hey @aido 👋

Unfortunately it is not up to us to plan reviews on these kind of PR, but rather @vforgeoux-ledger.
The review itself is likely to be performed by someone from the Donjon team.

As your app is a bit special vs. others, I'm not sure how updates are managed, but FWIW most 3rd party developer need to go through the app submission form for that.

[aido]

As your app is a bit special vs. others, I'm not sure how updates are managed, but FWIW most 3rd party developer need to go through the app submission form for that.

As suggested, Ledger Device App update form completed and submitted.

[aido]

Hey @fbeutin-ledger,

The Ledger Rule Enforcer checks seem to be failing with the same error for Stax and Flex:

/opt/flex-secure-sdk/lib_nbgl/src/nbgl_layout_keyboard.c:284:9: error: implicitly declaring library function 'snprintf' with type 'int (char *, unsigned int, const char *, ...)' [-Werror,-Wimplicit-function-declaration]
        snprintf(numText, sizeof(numText), "%d.", number);
        ^
/opt/flex-secure-sdk/lib_nbgl/src/nbgl_layout_keyboard.c:284:9: note: include the header <stdio.h> or explicitly provide a declaration for 'snprintf'
1 error generated.
make[1]: *** [/opt/flex-secure-sdk/Makefile.rules_generic:110: build/flex/obj/sdk/lib_nbgl/src/nbgl_layout_keyboard.o] Error 1
make[1]: *** Waiting for unfinished jobs....
scan-build: Analysis run complete.
scan-build: Removing directory '/__w/app-seed-tool/app-seed-tool/app-repository/output-scan-build/2025-08-28-112316-699-1' because it contains no reports.
scan-build: No bugs found.
make: *** [/opt/flex-secure-sdk/Makefile.rules_generic:216: scan-build] Error 2
Error: Check scan failed

Fail with 1 error(s) during Guideline_enforcer checks for 'flex'
Error: Process completed with exit code 1.

Is this an issue with the app or with the nbgl_layout_keyboard.c file in the SDK? I think it may be the SDK.