docs(spec): define fork-aware header chain engine

[evan-forbes]

Motivation

PR #229 exposed that the current single-chain header overlay cannot safely reconcile promoted forks across header sync, full-state resets, finalization, restart, and asynchronous completion boundaries.

This draft makes the replacement contract reviewable before implementation. It also captures the consensus boundary of a headers-only client: observable header validity is not proof of transaction or state validity.

Solution

Add a normative, fork-aware header-chain engine specification that defines:

• a durable header DAG with independent finalized, header_best, and verified_best frontiers;
• production consensus checks, settled network-upgrade pins, contextual validation, and deterministic greatest-work fork choice;
• mode-specific finality and bounded deterministic retention;
• generation-aware state transitions, body-failure evidence, VCT repair, and crash recovery;
• a versioned native v8 discovery protocol with exact auxiliary-record encoding; and
• a conformance, differential, property, crash, protocol, resource, audit-replay, and fuzzing matrix.

This document is the redesign oracle for PR #229; it intentionally does not preserve the current patch structure.

Testing

Documentation-only change.

• Confirmed the repository copy is byte-for-byte identical to the audited source.
• git diff --check
• Mechanical audit: 111 unique normative rule IDs, balanced code fences, and no TBD, TODO, or FIXME markers.

Specifications & References

• Follow-up to fix(sync): reconcile promoted header forks #229.
• Rendered implementation and testing plan.

Follow-up Work

Implement the engine in reviewable phases after the normative contract is accepted.

[v12-auditor]

Warning

No auditable source files found in this PR's diff.