diff: compare the cascade layer order the sheets declare

[samoht]

The tree walk paired two @layer blocks, found their bodies equal and reported nothing — even when the two sheets declare the layers in opposite orders and so resolve a conflict between them the opposite way.

@layer a; ahead of a @layer b block makes a the weaker layer. Drop that one statement and b becomes weaker instead: the same rules now win differently. The walk read the two sheets as identical.

It now compares the declared layer order and names the pairs that swapped:

Cascade layer order changed:
├─ b now precedes a
└─ order: a, b -> b, a

The order comes from Resolve.layered_rules, the model that already backs cascade_layer_precedence_rank — exposed as Resolve.layer_order rather than written a second time. Layers only one sheet declares are left to the existing added/removed reporting, so the comparison is over the layers both sides have. A layer declared inside @media/@supports/@container is not part of it, matching what Resolve.layer_order counts; that boundary is documented in both mlis.

Tests: the case that was pinned as a known gap is now a positive assertion, alongside a nested-sublayer case and a control — @layer a;@layer b; against @layer a,b; is one order in two spellings and stays quiet.

Tree_diff.t gains a layer_order field and Css_compare.stats a layer_order_swaps count, so a layer-order-only diff reads Changes: 1 swapped layer pair instead of falling through to "none classified structurally".