Add the lower-bound proof programme to the note

Section 4: un-stacking lemma (degree-3 removal preserves Phi, proved),
ear-peeling base case (k=0 => 2^(n-2)), reduction to the irreducible
case, and the irreducible lemma as the sole open conjecture (|Phi| >=
5/4 * 2^(n-2), tight at the degree-4/5 patch; wheel = floor(2^n/3) is not
extremal). Records the two dead ends (monotonicity false, universal
toggles insufficient) and ties each claim to its experiment.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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@@ -14,6 +14,7 @@ zonotope structure and the $2^{n-2}$ constraint floor}
\newtheorem*{prop}{Proposition}
\newtheorem*{conj}{Conjecture}
\newtheorem*{lem}{Lemma}
\newtheorem*{verify}{Empirical check}
\begin{document}
\maketitle
@@ -129,6 +130,87 @@ The minimal set is itself a sign-closed zonotope of size $2^{n-2}$, hull
dimension $n-2$, not a $\mathrm{GF}(3)$ subspace --- the same fingerprint
as $\S1$.
\section*{4. A proof programme for the lower bound}
The lower bound $|\Phi(D)| \ge 2^{n-2}$ reduces, by an exact
$\Phi$-preserving reduction, to a single lemma about ``irreducible''
disks. Two dead ends bound the search first: \emph{monotonicity is false}
--- inserting a degree-$4$ interior vertex can shrink $|\Phi|$ ($6\to5$,
$30\to28$; \texttt{monotonicity\_test.py}), so there is no reduce-to-base
proof by ``adding vertices only grows $\Phi$''; and \emph{universal
toggles are insufficient} --- a flip preserves feasibility for every
labelling only if it touches no interior vertex (a \emph{boundary-only}
face), and an irreducible disk can have none (the wheel has zero). What
does work:
\begin{lem}[Un-stacking; degree-$3$ removal preserves $\Phi$]
\label{lem:unstack}
Let $v$ be a degree-$3$ interior vertex of $D$, with link triangle
$abc$ and incident faces $(vab),(vbc),(vca)$. Its constraint
$\lambda_{vab}+\lambda_{vbc}+\lambda_{vca}\equiv 0 \pmod 3$ over
$\{+1,-1\}$ forces the three to a common value $s$, so each of $a,b,c$
receives $2s\equiv -s$ from $v$'s star. Let $D'$ delete $v$ and restore
$abc$ as one face. Then setting that face to $-s$ reproduces the
contribution $-s$ at $a,b,c$, and $s\mapsto -s$ is a bijection on
$\{+1,-1\}$. Hence the map is a bijection between feasible labellings of
$D$ and of $D'$ preserving every boundary value and interior constraint,
so
\[
\Phi(D) = \Phi(D'), \qquad k(D') = k(D)-1 .
\]
\end{lem}
\begin{verify}\textnormal{(\texttt{monotonicity\_test.py})} Degree-$3$
insertion gave exact equality in $8884/8884$ trials.\end{verify}
\begin{lem}[Base case; ear-peeling]
\label{lem:base}
If $D$ has no interior vertices ($k=0$) then $|\Phi(D)| = 2^{n-2}$. A
polygon triangulation has an \emph{ear} $(v_{i-1},v_i,v_{i+1})$ with
$v_i$ of face-degree $1$, so $\sigma_{v_i}=\lambda_{\mathrm{ear}}$ reads
the ear label directly; remove it and induct on the $(n-1)$-gon. The
boundary map is injective, giving $2^{n-2}$.
\end{lem}
\begin{prop}[Reduction to the irreducible case]
\label{prop:reduction}
Iterating Lemma~\ref{lem:unstack} terminates ($k$ strictly decreases) at
a residue $D^{\ast}$ with no degree-$3$ interior vertex and the same
$n$, and $\Phi(D)=\Phi(D^{\ast})$. The residue is either $k=0$, where
$|\Phi|=2^{n-2}$ by Lemma~\ref{lem:base}, or \emph{irreducible}: $k\ge1$
with every interior vertex of degree $\ge 4$. Hence
\[
|\Phi(D)| \ge 2^{n-2}
\quad\Longleftarrow\quad
|\Phi(D^{\ast})| \ge 2^{n-2}\ \text{for every irreducible } D^{\ast}.
\]
\end{prop}
\begin{conj}[Irreducible lemma --- the remaining content]
\label{conj:irreducible}
Every irreducible disk satisfies $|\Phi| \ge 2^{n-2}$; in fact
$|\Phi| \ge \tfrac54\cdot 2^{n-2} = 5\cdot 2^{n-4}$.
\end{conj}
\begin{verify}\textnormal{(\texttt{irreducible\_floor.py},
\texttt{wheel\_extremal.py})} Over $10^4{+}$ irreducible disks
($n=4,5,6$) there were $0$ floor violations and none sat on the floor.
The bound $\tfrac54\cdot 2^{n-2}$ is \emph{tight}, attained by a single
\textbf{minimal-degree} interior vertex (degree $4$ or $5$, which tie):
the ratio $|\Phi|/2^{n-2}$ rises monotonically with the interior vertex's
degree, $\tfrac54$ at $d\in\{4,5\}$, $\tfrac{21}{16}$ at $d\in\{6,7\}$,
$\ldots$, up to $\tfrac43$ at the wheel $d=n$, where
$|\Phi(W_n)|=\lfloor 2^n/3\rfloor$ exactly. So a proof of
Conjecture~\ref{conj:irreducible} should be stress-tested against the
degree-$4$ patch, the tight case --- \emph{not} the wheel.\end{verify}
\noindent
\emph{Status.} Lemmas~\ref{lem:unstack}--\ref{lem:base} and
Proposition~\ref{prop:reduction} are proofs; they settle every disk that
un-stacks to $k=0$ (the entire Apollonian class). The whole open content
is Conjecture~\ref{conj:irreducible}, with guaranteed $25\%$ slack and a
single explicit extremal disk.
\section*{Consequence for the pigeonhole}
Even a maximally-constraining child still presents $2^{n-2}$ outer