Avoid recursive root lowering in make

[quinnj]

Summary

• Limit automatic root lowering to sources for which structlike(style, source) is false.
• Preserve the @nonstruct root mapping added by Fix root lowering for non-struct mappings #64.
• Allow struct-like lower methods to call make for field traversal without recursion.
• Document the root-lowering contract and add nested regression coverage.

Root cause

PR #64 made the generic structural make path call lower for every root source. A struct-like lower method implemented with make therefore entered a direct make -> lower -> make cycle.

The fix keeps root representation lowering for non-struct sources, which is the case required by #29, while traversing ordinary struct roots directly. applyeach continues to lower nested values.

Validation

• Julia 1.9.4: full package suite passed, 255 tests.
• Julia 1.12.6: full package suite passed, 255 tests.
• Julia 1.12.6: JuliaC trim suite passed, 7 checks.
• Julia 1.13.0-rc1: full package suite passed, 255 tests.
• Documenter build and doctests passed.
• Focused four-field struct-to-dictionary check retained 576 bytes per call and showed no measured slowdown.

Fixes #66

Co-authored by Codex