spec(port): port the ratchet tree blank node depth guard (Refs #8883)

[dmitrii-f-t27]

The twenty-third CR-CHAT-03 ring target (W3, tracker #8883, epic gHashTag/trios#1812): port of crates/trios-chat/rings/CR-CHAT-03/src/ratchet_tree_blank_node_depth_guard.rs (Wave-75 Lane A, R-CHAT-2).

TreeKEM resolution skips blank nodes; a deep run of consecutive blanks leaves a subtree unresolvable — no member can encrypt to it, and an update path through the chain blocks entirely. Rules, in the original's fixed order:

1. tree non-empty,
2. per node: depth ≤ 16; a leaf is never blank; the consecutive-blank run ≤ 4 (a non-blank node resets the run),
3. after the walk: the root not blank.

Carried as the D5 verdict with every payload field: the measured run of BlankTooDeep(usize). Honesty note: the prose lists a non-blank-after-chain rule and the enum declares NonBlankAfterBlank, but no code path ever answers it — the run reset is silent; the variant stays in the enum, declared and unreachable exactly as in the original.

All 10 original RBND tests transcribed with exact asserts (the five-blank chain, the max-depth boundary of exactly four, the chain-reset with two runs of two), plus a rule-order ladder (the run rule answers over the root rule on a five-blank chain ending in a blank root) — 11 total.

Checks (conventions 0-14): t27b test 11 passed 0 failed, 0 blockers; t27c test-report 11/11, 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. Twenty-third CR-CHAT-03 target (23/47 T-PORT) — requesting owner merge when convenient, Refs #8883.

[dmitrii-f-t27]

Superseded by #10175: the remaining CR-CHAT targets now land as one port-wave batch per specs/policy/pr_budget.t27 (#10067/#10074), which asks for one PR per ring batch instead of one per file. The spec body, seal and ledger row in this PR are unchanged there; the batch also carries one self-consistent duplicate-body bless. Closing this one-file PR keeps the merge queue and the Actions queue small.