Initial JS runtime and Arborix Implementation

ext/bundle-runtime-profile-v1.md

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+1. Scope
+   This profile defines the minimum required behavior for runtimes that execute tricu bundles.
+
+2. Non-goals
+   No tricu source parsing.
+   No lambda elimination.
+   No module system.
+   No package manager.
+   No local DB requirement.
+   No authoring names beyond bundle exports.
+
+3. Required bundle sections
+   Header
+   Manifest/exports
+   Merkle nodes
+
+4. Optional/skippable sections
+   Source, debug, package metadata, signatures, provenance, etc.
+
+5. Entrypoint selection
+   Explicit export name first.
+   Else export named main.
+   Else single default root.
+   Else error.
+
+6. Node payload format
+   Leaf/Stem/Fork byte layouts.
+
+7. Hash verification
+   Domain string and payload hashing rules.
+
+8. Closure verification
+   All referenced child hashes must exist.
+
+9. Runtime representation
+   Suggested JS representation, but not normative.
+
+10. Reduction semantics
+   The six Tree Calculus apply rules.
+
+11. Codecs for v1
+   Raw tree required.
+   Maybe string/bool optional or experimental.
+
+12. Required error cases
+   Bad magic/version, missing export, hash mismatch, malformed payload, missing child.
+
+13. Test fixtures
+   List of bundles the implementation must pass.