docs(impl): isolate global HIR type validation

[sanohiro]

Summary

• isolate the independently complete am-g-t global HIR type-domain validation slice
• define exact concrete-root, template-reachability, reference, width/lane, and inline-cycle rules
• block all remaining placement, nominal/link, callable namespace, declaration/header, and body work behind a new am-r public-contract ledger and fresh review
• record why the 1,535-line combined checkpoint was split under the existing size and closure-matrix rules

Review closure

The initial post-open reviews found that the broader plan omitted per-position type admissibility and the complete logical/emitted/generated callable namespace, and that exact compiler-symbol rejection would silently change current source acceptance. The plan was reopened rather than patched: none of those surfaces is authorized for implementation by this PR.

The fresh rescope review found one root-predicate ambiguity. The final contract now makes every non-template tagged/function node a concrete root and excludes only the transitive Param-dependent abstract template graph. That finding is closed and the final review is clean.

Verification

• git diff --check origin/main...HEAD
• fresh independent adversarial rescope review: CLEAN
• final scripts/pre-pr.sh HEAD/base-bound attestation