[dmitrii-f-t27]

The eighth CR-CHAT-03 ring target (W3, tracker #8883, epic Trinity-S-AI-DNA/trios#1812): port of crates/trios-chat/rings/CR-CHAT-03/src/external_proposal_origin_unbound.rs (Wave-33, L-CHAT-3-epou, R-CHAT-11, RFC 9420 12.1.8.2/6.2).

An external proposal is authored outside the membership — by a declared external sender or a prospective new member — and carries its origin. The gate rejects: a phantom origin the group never declared, a kind the origin may not submit (new members may only Add and external-init per RFC 9420 12.1.8.2; Update is forbidden from every external origin), a cross-group or cross-epoch splice, a replayed (origin, proposal_id) pair, a non-canonical signature length, and the all-zero Ed25519 sentinel. Rules, in the original's fixed order:

1. origin_signature length == 64 (Ed25519, RFC 8032 5.1.6),
2. origin ∈ declared_external_origins,
3. (origin, kind) ∈ permitted_kinds_for_origin,
4. group_id == expected_group_id,
5. proposal_epoch == current_epoch,
6. (origin, proposal_id) ∉ used_external_proposals,
7. origin_signature not all-zero.

Carried as the D5 verdict — all seven original error variants carry no payload, so the code is the whole verdict.

Adaptations (t27 conventions): proposal/view structs → flat parameters; the ExternalOrigin enum (one carrying variant) → kind enum + record holding the sender index (unread by the unit kinds); declared-origins and permitted-pairs vectors → slices of records; the used-proposals vector → slice of records holding the origin and the proposal identifier at its declared maximum capacity (255) with the length; the original's three contains probes answered by scans.

All 10 original EPOU tests transcribed with exact asserts (the canonical acceptance is the ExternalSender-Remove path, the only one permitted Remove), plus 6 helper tests and a full rule-order ladder stacking each earlier rule's violation on the later ones (17 total).

Checks (conventions 0-14): t27b test 17 passed 0 failed, 0 blockers; t27c test-report 17/17, 0 vacuous; gen-rust output compiles under rustc (0 'static str, 0 Vec<const); coverage Untested 0; ASCII only; seal saved, all hashes MATCH; ledger row added to docs/reports/t27b_expectations.json.

Refs #8883

[dmitrii-f-t27]

All required checks green (validate, check-linked-issue, parse-ratchet, t27b-native-linux, t27b-native-macos); cli-tri / duplicate-bodies / t27b-native-ratchet are the known non-required reds. Steward READY. Eighth CR-CHAT-03 target (8/47 T-PORT) — requesting owner merge when convenient, Refs #8883.