[ChrisRackauckas-Claude]

Summary

• Add definition-point SciMLStyle documentation for the exported tree, iterator, invariant, partition, representation, and time-integration APIs.
• Document the supported tree and iterator contracts in docs/src/developer_interfaces.md, including the deliberate SubtreeIterator limitation and mutable-buffer ownership rules.
• Add generic consumer tests for rooted and bicolored trees and an export-wide definition-point documentation check.

This is a focused docs/interface follow-up to #240. It does not change compat metadata, runtime implementations, or the existing QA exception for the extension-local _distinguishable_colors hook. Please ignore this draft until reviewed by @ChrisRackauckas.

Verification

• Runic check: passed for all tracked Julia files.
• typos: passed on changed files.
• git diff --check: passed.
• Generic interface tests: iterator 37/37, tree 15/15, exported-doc check 1/1.
• GROUP=QA julia --startup-file=no --project=. -e 'using Pkg; Pkg.test()': QA/qa.jl 21/21.
• GROUP=Core julia --startup-file=no --project=. -e 'using Pkg; Pkg.test()': Core interfaces 53/53, order conditions 166/166, rooted-tree tests 8590/8590; the existing colored-tree suite reports 571975 passes and 6 pre-existing broken tests.
• Local-source docs build with Pkg.develop(Pkg.PackageSpec(path=pwd())): passed through doctests, cross-references, checks, and HTML rendering.
• Exported binding scan: 37 exports, 0 undocumented.
• The six existing colored-tree broken tests are unchanged and are not suppressed by this PR.