fix: trailing anchor constraints trigger backtracking

[zharinov]

Summary

• Trailing anchor constraints (. )) now trigger backtracking when the first match isn't the last child
• Uses skip-retry pattern (same as quantifiers) to retry with next sibling until finding one that IS last
• Adds 4 engine tests covering trailing anchor scenarios

Why

Queries like (program (function_declaration) @last .) failed to find the last function when multiple existed. The compiled bytecode had no checkpoint for retrying when UpSkipTrivia failed after a search match succeeded.

Notes

The fix adds compile_named_node_with_trailing_anchor() helper that generates:

entry: Match(nav, node_type) → down_wildcard
down_wildcard: Match(Down, wildcard) → try
try: epsilon → [body, retry_nav]          ← checkpoint
body: items (StayExact) → up_check
up_check: Match(UpSkipTrivia) → exit
retry_nav: Match(Next, wildcard) → try    ← advance on failure