fix(vm): a text run from inside a line no longer takes the line's place -- LOAD and THRU go on with the rest of it

FABRIC-3.md XXXIV.1, traced and fixed.  vm_interpret() copied each text
into the VM's one input buffer and reset the position.  LOAD runs its
block by calling it, so the block replaced the line LOAD was on and
nothing after LOAD or THRU on that line was ever interpreted; the same
for -->, a capsule's EXEC and a message kernel-Hermes delivers.
vm_interpret() now keeps what the outer text had not yet read when it is
called from inside another, and puts it back after, as FORTH-79's LOAD
keeps the input stream it interrupts.  In the kernel's VM and the hosted
one alike.

POST: LOAD.rest_of_line (1 LOAD 0 0 /) and THRU.rest_of_line
(1 2 THRU 0 0 /) failed without the fix (logs/20261014-061222) and pass
with it.  Three-ISA acceptance: logs/20261014-061508 (amd64), -061624
(aarch64), -061837 (riscv64), 1052 tests, none failed, zuse prompt,
hash 0x08873e0f44b7cb2a unchanged.  The hosted binary: 100 LOAD 1 + .
gives 8.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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