Fix -Werror=implicit-fallthrough build failure in crafting.c

[kovalancik06-max]

Summary

• Fixes Bug report: statement may fall through #194 — ESP-IDF/PlatformIO builds fail with -Werror=implicit-fallthrough on two spots in getCraftingOutput() in src/crafting.c.
• Both fallthroughs are intentional (slab-recipe case group falls into the shovel/sword case group, and the axe-recipe break-guard falls into the helmet case group, so shared items like oak_planks/cobblestone get checked against both recipe sets). The fix annotates them with __attribute__((fallthrough)); instead of adding a break;, since a bare break (as suggested in the issue thread) would silently remove the shovel/sword/helmet recipes for wood, stone, and leather items.
• __attribute__((fallthrough)) is a GCC/Clang extension independent of the C standard version, so it works under this project's C89-compatibility goal (per the discussion in Convert all source code to C89 #81 about not using C23's [[fallthrough]]).

Test plan

• Reproduced the exact reported errors (error: unannotated fall-through between switch labels at both call sites) by compiling the pre-fix crafting.c with -Wimplicit-fallthrough -Werror.
• Confirmed the patched file compiles cleanly with the same flags against the project's real headers (stubbed only the registries.h enum, which is normally generated from a vanilla server jar).
• Diff is 2 lines, no behavior change on the taken branches — only silences the fallthrough diagnostic on the paths that already fell through.