Interior anchor after a skippable first item is ignored when the sequence also has a trailing anchor

[zharinov]

Problem

A sequence can have an optional first item, an anchor, and a trailing anchor: {(a)? . (b) .}. The anchor rules are not enforced once the sequence has that trailing ..

Q='Q = (program {(lexical_declaration)? @a . (debugger_statement) @b .})'

cargo run -p plotnik -- run -q "$Q" -s 'debugger;'             -l javascript
# Want: match {a: null, b: debugger}  (a skipped, b is the first and only child)
# Got:  no match — exit 1

cargo run -p plotnik -- run -q "$Q" -s 'foo; debugger;'        -l javascript
# Want: no match  (a skipped, so b must be first; it isn't)
# Got:  {a: null, b: debugger} — exit 0

cargo run -p plotnik -- run -q "$Q" -s 'let x; foo; debugger;' -l javascript
# Want: no match
# Got:  {a: null, b: debugger} — exit 0

Reproduces on master. This is a different code path from #415, so #415's fix does not touch it.

Cause

A sequence with a trailing anchor goes through compile_named_node_with_trailing_anchor (crates/plotnik-compiler/src/compile/expressions.rs). It works differently from the normal sequence path:

• It compiles the body with Nav::StayExact and drives matching with an outer loop: go to the first child, then step to each next sibling and retry.
• The body's first nav is StayExact, not a Down, so the skip-exit logic from Interior anchor is silently dropped after a skippable first item #415 never kicks in. When the optional is skipped, the follower steps forward instead of staying put.
• The outer loop also tries the body at every child position. So even with the right nav, it would still find b at a later position, which breaks the "b must be the first child" rule.

Fix

The trailing-anchor path needs the skip-exit split (match path: b is a sibling; skip path: b degrades to the first-child anchor) and must not search forward past the first child on the skip path.

This sits in #419's territory (it lists the trailing-anchor retry as a search mechanism to unify), but the leading-anchor part is specific, so track it here.

Acceptance

• The repros above behave per the Want lines.
• Tests cover, for a trailing-anchored sequence with an optional first item: item present + adjacency holds; item present + adjacency broken; item absent + b is first-and-last; item absent + b not first.
• Strict (.!) variants too.