S1: Remove silent fallbacks from scripts/build.sh

Stub-burndown register S1 (P0), blocks Phase 0 truth.

• Default path references nonexistent kernel/src/kernel.zig
• rpi5 path references nonexistent rpi5_main.zig
• unified path silently substitutes an hlt-loop stub (kernel_stub.s) when the compiler is missing

Per MASTER_PLAN Phase 0 exit gate: zero silent fallbacks — missing compiler or failed compile must exit nonzero. Related to #8. Part of #21.