ordinal(buchholz): rank-lex-jb scaffold — parallel rank with richer bplus second component

[hyperpolymath]

Summary

Opens the sole-viable forward path for the bplus-chain-level bpsi-source-at-equality discharge after PR #146's checked refutations of the two prior unblock routes (additive-principal on generic sums + strict-left-mono of _⊕_).

New module Ordinal.Buchholz.RankLexJointBplus:

• leftmost-α : BT → Ord — leftmost-ψ-α-rank discriminator.
• rank-lex-jb : BT → RankLex — parallel lex rank with richer bplus second component (leftmost-α instead of oz).
• rank-lex-jb-strict-second-at-equal-first — load-bearing scaffold primitive: fires <lex-second at equal first components.
• 4 + 4 definitional sanity lemmas; leftmost-α-bplus-left two-level convenience.

Honest scope

Scaffold-only. The full headline (joint-bplus <ᵇ-+1 discharge at bpsi-source-at-equality) requires the (a)+(b)+(c) assembly documented in the module preamble:

• (a) equal-first lex-second leg — closed here.
• (b) strict-first lex-first leg — multi-PR follow-on (same bplus-chain sum-bound challenge that motivated the refactor).
• (c) first-component trichotomy — non-trivial Ord-valued decidability bridge not landed in Phase13 yet.

The next session's task is (b) + (c) + assembly into a full headline; this scaffold lays the foundation.

Test plan

• agda --library-file=/tmp/echo-libraries -i proofs/agda proofs/agda/Ordinal/Buchholz/RankLexJointBplus.agda exits 0
• agda --library-file=... proofs/agda/Ordinal/Buchholz/Smoke.agda exits 0
• scripts/kernel-guard.sh PASS (kernel cone funext-free + classification in sync)
• Wired into All.agda adjacent to RankLexSlice3; pinned in Ordinal/Buchholz/Smoke.agda under own block per CLAUDE.md Working rules
• Zero postulates, --safe --without-K, no funext

🤖 Generated with Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 74 issues detected

Severity	Count
🔴 Critical	17
🟠 High	36
🟡 Medium	21

⚠️ Action Required: Critical security issues found!

View findings

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