Standardize LaTeX math notation: replace pmatrix with bmatrix across all lecture files

[Copilot]

This PR standardizes LaTeX math notation throughout the lecture files by replacing all instances of pmatrix with bmatrix to align with the QuantEcon style guide.

Changes Made

• 7 lecture files updated with consistent bmatrix notation
• 43 total replacements of \begin{pmatrix} → \begin{bmatrix} and \end{pmatrix} → \end{bmatrix}
• All LaTeX syntax remains properly balanced and valid

Files Modified

• lectures/svd_intro.md (3 instances)
• lectures/stats_examples.md (1 instance)
• lectures/lagrangian_lqdp.md (13 instances)
• lectures/multi_hyper.md (3 instances)
• lectures/opt_transport.md (7 instances)
• lectures/newton_method.md (13 instances)
• lectures/lake_model.md (3 instances)

Example Change

Before:

X = \begin{pmatrix}
    U_{t+1}/N_{t+1} \\
    E_{t+1}/N_{t+1}
\end{pmatrix}

After:

X = \begin{bmatrix}
    U_{t+1}/N_{t+1} \\
    E_{t+1}/N_{t+1}
\end{bmatrix}

The changes are purely stylistic and maintain all mathematical content while ensuring consistency with the established style guide. All matrix expressions now use square brackets (bmatrix) instead of parentheses (pmatrix).

Fixes #314.

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[mmcky]

Here is the relevant style guide page

https://manual.quantecon.org/styleguide/math.html

[github-actions]

🚀 Deployed on https://689acc9ad8b1fe2f5ee3d06e--nostalgic-wright-5fa355.netlify.app

[mmcky]

This PR updates the bracket style for matrices to use square brackets.

• this issue should be copied to other lecture repositories for the style to be unified across the lecture series.

[jstac]

Thanks @mmcky

[mmcky]

@bishmaybarik would you have time to look through the diff of this PR and cross-check to see if these changes are consistent and make sense in context? I have had a pretty good look myself and we can discuss when we link up on Thursday.

[bishmaybarik]

Hi @mmcky, I'd love to go through the PR and flag any points that is not consistent. I'll let you know by Thursday during our meeting.