GGUF export path is under-tested: 5 files below the 90% coverage floor

[chrishayuk]

The gguf-export-abi work (#363) landed ~2,100 lines with 4 inline tests between
five files, putting all five below the per-file 90% coverage floor and turning
larql-vindex's coverage gate red on main.

file	lines	inline tests	coverage
gguf/export.rs	419	1	50.84 %
gguf/emit/mod.rs	517	0	83.09 %
gguf/vocab.rs	400	3	83.44 %
gguf/walk/mod.rs	434	0	88.52 %
gguf/preflight/mod.rs	346	0	88.89 %

Why this needs repaying rather than baselining permanently

A permanently-red coverage gate destroys the ability to notice a real
regression — the next genuine coverage drop becomes indistinguishable from this
noise. As an interim measure these five have been added to
crates/larql-vindex/coverage-policy.json at their current levels, so the
gate is green today and any drop BELOW today still fails. That is a ratchet, not
an exemption: the entries are marked as debt and dated, and should be raised as
tests land, not lowered.

export.rs at 50.84 % is the priority — half the export path is unexercised.
emit/, preflight/ and walk/ have no inline tests at all.

Not part of this

Two represent/kda_candidate* entries were added in the same change for a
different reason and are NOT debt of this kind: kda_candidate_real.rs is a
#[cfg(test)] driver gated on a real 48 B container CI does not have, and
kda_candidate.rs carries gpu-gated paths. Those are structurally unreachable
in the ubuntu coverage job.