Add native Binder checks for LEGO formatting/unit/prose contracts, extend mlsysim formatter helpers, and normalize Vol1/Vol2 LEGO output strings through typed formatters.
Validate precision-sensitive prose rendering across both volumes and promote direct math/string assembly to fmt_math, fmt_display_math, fmt_text, and domain-specific helpers.
Adds 'binder check cli --scope binder-canonical' (default=yes): parses
.pre-commit-config.yaml and fails if any local hook scoped to book/quarto/
contents/ calls a raw script instead of ./book/binder. Repo-wide guards, CI
hygiene, third-party hooks, and other subprojects are not book content and
are not inspected; an explicit ALLOWLIST covers any justified exception.
This makes Binder-as-single-front-door a mechanically enforced invariant
instead of a convention — the day someone wires an off-Binder book hook (as
book-check-lego-units once was), the commit fails with an actionable fix.
Runs via the existing always-run book-check-cli-contract hook; no new hook.
Also hardens the cli-contract 'check cli help' case to assert on stable
tokens (group header + scope names) rather than the note phrase, which Rich
wraps once a group gains a scope. Extends it to cover the new scope.
