The tie control region is not a VIF or GIF tag stream; and vert_ofs is mis-named

Went after the GS's GIF packets in the tie AD-GIF region. Got a negative result,
a new universal invariant, and a retraction of my own first probe, which reached
the right answer for the wrong reason twice.

THE NEGATIVE RESULT

Tried the leading `5 * shader_count` quadwords of each packet (the
`bfc_distance` field's stated meaning) as VIF tags -- command in bits 0-6, size
in 7-14, mark in 15 -- and as GIF tags -- type in the top byte of word 1, length
in bits 0-2 of word 0. Both uniform. Over 13,042 records: 10,443 distinct values
of word 0, no GIF tag type above a count of 2. That is the control result rule 4
asks for, and it says stop.

THE RETRACTION

The first probe scanned `bfc_distance` quadwords, which for most records
overruns `vert_ofs` and runs into the vertex data. Pooling a control region with
vertex positions -- two populations with different structure -- gives a uniform
answer, so "not a tag stream" came out right from a measurement that was
measuring the wrong bytes. That is exactly the rule 3a failure, and I only caught
it because the hex dump I took next looked structured while the histogram said
random.

Re-measuring quadword 0 alone, on its own population, gives the same uniformity.
The conclusion survives; the reasoning did not, and both are recorded.

THE PARTITION, AND A NEW INVARIANT

The hex dump showed the packet layout directly:

    q[0 .. vert_ofs)            control        3..8 quadwords, peak at 5
    q[vert_ofs .. +vert_size)   vertices
    q[vert_ofs+vert_size ..]    scissor

and `scissor_ofs == vert_ofs + vert_size` holds on **15,219 of 15,219** records
across all 27 levels, including every one whose strip_count is non-zero. That is a
derived quantity stored explicitly, and it holds everywhere -- which is the check
that confirms the three regions really are laid out the way the existing packet
length formula assumed. It is now a hard check in invariants_hold(), which is
something the previous four could not be.

The fixture in tie.rs failed the moment I added it: it had vert_ofs 4 and
vert_size 0x20 with scissor_ofs 0x30, which is 4 + 0x24. The synthetic data was
never constrained by a relation that did not exist yet. Fixed to 0x24.

vert_ofs IS MIS-NAMED

It is not a vertex offset. It is the *length* of the leading control region, and
the vertex data begins at that many quadwords into the packet. The field name
stays because the on-disc order is fixed and everything already written about this
record says vert_ofs, but the doc now states what the value is and gives the
distribution.

STILL UNKNOWN

What the control region contains. Not standard DMA tags; possibly pre-merged GS
register writes in another framing, or per-class state rather than per-primitive
draw data. Recorded as an open question rather than guessed at.

23 + 19 + 58 + 156 + 22 = 278 passing with the disc present (disc binary 35.67s),
clippy clean.

docs/roadmap.md

@@ -742,7 +742,12 @@ lookup, and it is the main thing inherited from ReRAC.
 - [ ] VU0/VU1 skinning and lighting on the PS2 float model
 - [ ] LOD/morph/cull per `TfragProc` and `TieProc`
 - [ ] Occlusion culling, fog, shadows
-- [ ] GIF/VIF packet stream interpretation
+- [~] GIF/VIF packet stream interpretation. **The tie control region is not a VIF or GIF
+      tag stream** -- measured uniform on both framings across 13,042 records, so
+      the GS's draw packets are not stored there in standard DMA form. What the
+      probe did establish is the packet partition (control / vertices / scissor)
+      and a universal invariant, `scissor_ofs == vert_ofs + vert_size`, on 15,219
+      of 15,219 records. See `docs/plan/level-assets.md`.
 
 **Gate:** CPU and GPU paths agree on every vertex of a reference level — the
 same identity check ReRAC runs.