types ratchet red on master: #7216 gave Tree a second definition (t27b conformance spec) #7294

Problem

Master's Corpus Ratchet workflow (job "Corpus ratchet (expected-failure ledger)", step "A type name may not gain a second definition") has been red since #7216 (0a05923):

ledger 84 name(s), observed 85
  + Tree  NEW conflict

tri types dup names the two definitions:

Tree  CONFLICTED  (2 definitions)
    specs/tri/t27b/conformance/unresolved_signature.t27:18
        kind: u32, kids: [KIDS]Tree
    specs/tri/trees/tree.t27:13
        (newtype): T

#7216 added the first one. Its own master run (37568156807) was cancelled, so the first master run to measure it was 37568215828 on df00ec4 (#7242), and every PR based on master since then carries this red. One example is #7291, whose run 37569428325 has the same single line. The check is not required, so it does not block merges, but it hides any new conflict behind a standing red.

Expected

The conformance spec's struct is a stand-in for "a struct that holds itself by value", so its name is arbitrary. Rename it, for example to SelfTree, in specs/tri/t27b/conformance/unresolved_signature.t27 together with whatever t27b expectations name it. Do not bless Tree into the ledger: two unrelated definitions under one name are the defect #2774 describes.

Criteria

• tri types ratchet exits 0 on master with the ledger unchanged at 84 names.
• tri types dup | grep -c '^ Tree ' prints 1.
• The t27b conformance run for unresolved_signature.t27 gives the same verdict as before the rename.

Refs #7216

Boundary

• specs/tri/t27b/conformance/unresolved_signature.t27
• specs/tri/trees/tree.t27

User Scenarios

• Given the Corpus Ratchet workflow runs on master, When tri types ratchet executes the "A type name may not gain a second definition" step, Then it exits 0 with the ledger unchanged at 84 names.
• Given the type duplicate check runs, When tri types dup is invoked, Then it reports exactly one definition for Tree and zero conflicts for that name.
• Given the t27b conformance spec unresolved_signature.t27 is renamed from Tree to SelfTree, When the conformance suite runs that spec, Then it produces the same verdict as before the rename.

Requirements

• FR-001: The struct named Tree in specs/tri/t27b/conformance/unresolved_signature.t27 MUST be renamed to SelfTree (or another non-conflicting name) along with all references within that file.
• FR-002: Any t27b expectation files that reference the old Tree name in the context of unresolved_signature.t27 MUST be updated to the new name.
• FR-003: The definition of Tree in specs/tri/trees/tree.t27 MUST remain unchanged.
• FR-004: The tri types ratchet ledger MUST remain at 84 names with no new entries added.

Success Criteria

• tri types ratchet exits 0 on master with the ledger unchanged at 84 names.
• tri types dup | grep -c '^ Tree ' prints 1.
• The t27b conformance run for specs/tri/t27b/conformance/unresolved_signature.t27 gives the same verdict as before the rename.