- Engine:
* EnterDigit now accepts A-F (uppercased) only for BaseHEX; decimal path now validates 0-9.
* EnterDecimalPoint ignored for non-DEC.
* currentNumericValue parses entry using current base (ParseInt base 16 for HEX, extended to BIN/OCT for robustness).
* FormatForDisplay prefers returning the raw (upper) entry string when active *and* base indicator matches (so typing 1A in HEX shows '1A', not 26).
* CycleBase now commits any active entry (parsed with old base) before switching. This makes mid-entry Tab safe and preserves CERR semantics.
- UI: added A-F/a-f key handling in Update (only when CurrentBase()==HEX). Sets pressedKey (harmless since no visual A-F buttons in tight scope).
- Tests: added 4 focused tests for hex digit acceptance, decimal ignore, mode gating, and mid-entry base switch + subsequent arithmetic.
- All existing behavior (DEC, CERR, etc.) preserved.
- Build + tests green.
This is the minimal 'keyboard only while indicator says HEX' scope requested. No visual A-F keypad buttons, no fractional hex, no changes to BIN/OCT entry.
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