Stage A: fix USE's silent stack-underflow/bad-address guards, and the Hera fault-scoping gap they exposed (FABRIC-3.md §XXXII.1)

mama_word_use()'s two silent vm->error=1 guards (dsp<1 stack underflow,
NULL from vm_ptr()) now print a diagnostic and return, matching the
function's other five guards.

Live verification of that fix alone surfaced a bigger problem: with the
guard no longer silent, the REPL proceeds to interpret the leftover
token as an unrecognized word, which independently sets vm->error, and
sk_repl_step()/sk_repl_run()'s Hera-branch still hard-halted on that.
Investigated kernel_main.c's boot/capsule-load paths directly: they
already catch and clear mama->error entirely separately, before
sk_repl_run() is ever entered -- so the "no fallthrough surface" halt
in these two REPL functions was never protecting a boot-time fault, only
an ordinary interactive REPL-turn one. Both functions now recover
unconditionally on any VM's error, Hera included, matching how a
redirected (WIREBIND/USE'd) identity's session already recovered.
Removed the now-fully-unused sk_fault_handler().

Verified live, amd64: USE rajames at Zuse's own console prints the new
diagnostic, then "VM fault -- session recovered, resuming", console
stays interactive afterward. Three-arch clean qemu acceptance passed
before and after the Hera-fault-scoping change.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BWpNjdwPtFLuVLaAq44L9K

experiments/bare_metal/latest/amd64.csv

@@ -1,2 +0,0 @@
-tick_number,elapsed_ns,tick_interval_ns,cache_hits_delta,bucket_hits_delta,word_executions_delta,hot_word_count,avg_word_heat_q48,window_width,actual_window_size,predicted_label_hits,jitter_bits,apic_ticks,time_trust_q48,variance_q48,vm_call_depth_max,hera_heat_q48,hermes_heat_q48,artemis_heat_q48,fleet_k_q48,fleet_conserved,switch_count_cumulative,switch_current_slot,switch_hera_readiness,switch_hermes_readiness,switch_artemis_readiness,switch_ticks_since
-104,1040000,890000,0,0,15665,80,42,256,256,0,4695806354533646336,405,65536,0,1,65536,0,0,65536,1,0,0,0,5,5,5