fix(lean): navigation chain round 2 — headers, footer, README

[jsboige]

Summary

Round 2 of Lean navigation chain fixes following the comprehensive audit.

Fixes

Notebook	Issue	Fix
Lean-14-Conway-Tribute	Header: prev=12 (wrong), two >> next arrows	prev=13, single next=15
Lean-15-Kochen-Specker	Header: prev=12 (wrong), no arrow markers	prev=14, terminal node
Lean-7-LLM-Integration	No footer navigation (dead-end)	Added footer nav cell
Lean-1-Setup	Serie complete stops at entry 9	Updated to 15+7b entries
README.md	Lean-14 missing from catalog, pedagogical_count=16	Added Lean-14 everywhere, count=17

Scope

Markdown-only changes (navigation cells, series list, README). No code cells modified. Per rule C.3, no Papermill re-execution needed.

Verification

Post-fix audit confirmed:

• Lean-14: header matches footer (prev=13, next=15)
• Lean-15: header matches footer (prev=14, terminal)
• Lean-7: footer nav cell present (37 cells total)
• Lean-1: 15 series entries + 7b = complete coverage
• README: pedagogical_count=17, Lean-14 in all tables/trees

[github-actions]

Notebook PR Validation: PASS

• Notebooks checked: undefined
• Code cells validated: undefined
• Result: All passed

Checks: H.1 (no errors), H.3 (execution_count), C.1 (no banned patterns)
Non-Python kernels (.NET/Lean): C.1 + errors only (execution_count advisory)
QuantConnect notebooks: C.1 + errors only (require QC Cloud for execution)