Register Hestia for switch signals -- FABRIC-3.6.md task 1.5

Added a fourth capsule_vm_find_by_name_nocase("Hestia", ...) +
sk_vm_switch_signal_register(...) block in src/starkernel/kernel_main.c,
same shape as the existing Hermes/Artemis blocks, placed after all four
fleet members are confirmed born -- the existing comment on this block
already states why: no critical-section protection during setup, so
registering earlier risks the signal firing mid-birth.

Three-arch boot clean: amd64/aarch64/riscv64 all reach [zuse@Hera] ok>,
zero UNKNOWN WORD, hashes identical to task 1.4's baseline (this is
pure C runtime state, doesn't touch any FORTH dictionary). No compiler
warnings.

Took FABRIC-3.5.md SXXXV.0's "invisible by default" warning literally
rather than trusting a clean boot log alone: SXXVIII.2's own recorded
switch-storm signature is "QEMU pinned near 100% CPU, serial log frozen
solid," not an error message. Confirmed normal wall-clock boot time on
all three (~30s) and, since TCG itself always shows ~100% CPU
regardless of guest workload, watched each serial log's line count at
the idle prompt for 5-10s and confirmed it stopped growing rather than
flooding or silently stalling.

Authorized by Captain Bob ("yes").

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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