Move PLOT/FB-WIDTH/FB-HEIGHT registration to Hestia only -- FABRIC-3.6.md task 1.8

Real fix for task 0.8's finding. register_framebuffer_words() was
called unconditionally from register_forth79_words() (word_registry.c),
itself called unconditionally from vm_init_with_host() -- the generic
per-VM bootstrap every VM goes through, with no way to know a VM's
name at that point.

Removed the unconditional call. Added a name-gated call instead in
capsule_birth_baby() (capsule_birth.c), beside the existing
is_fleet_foundation check -- the one place in the birth path where
capsule_name and the newly-allocated VM* are both in scope together:
Hestia gets register_framebuffer_words(), nobody else does.

Positively verified live, exactly as the task's own check demands:
FB-WIDTH via VM-EXEC returns UNKNOWN WORD in Hermes and Artemis, 1280
in Hestia. Confirmed at the fundamental level too: Hera's own base
PARITY:M7.1a word count dropped from 531 to 528 -- exactly the three
words removed -- identical across all three architectures.

Three-arch boot clean: amd64/aarch64/riscv64 all reach [zuse@Hera] ok>,
zero UNKNOWN WORD during boot. mkcapsule --lint capsules/ clean, 38
files / 0 violations (pure C change, no capsule content touched). No
compiler warnings.

Side effect on the vendored hosted build, expected and not a
regression: capsule_birth.c is kernel-only, so the hosted starforth
binary has no Hestia concept and now never registers these words at
all -- confirmed live. framebuffer_words.c's own top comment already
calls this surface "kernel-only, no-op on hosted builds," so the prior
stub registration was already vestigial. Ran a plain `make` sanity
build per CLAUDE.md's own stated purpose for that target; regenerated
lfs/amd64/starforth included here rather than left stale.

Authorized by Captain Bob ("yes").

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

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