MAINT: Phase 1 - Add HTML archives to GitHub release assets (#659)

Implements Phase 1: HTML archive backup on releases.

## Changes

This commit adds steps to the publish.yml workflow to create and upload HTML archives as GitHub release assets:

1. Create HTML archive - Compresses _build/html/ into a .tar.gz archive after the HTML build
2. Generate checksum - Creates SHA256 hash for integrity verification (html-checksum.txt)
3. Create manifest - Generates metadata file with build information (html-manifest.json)
4. Upload to release - Attaches all three files to the GitHub release using softprops/action-gh-release@v1

## Release Assets Created

Each publish-* tag will now include:

• 📦 lecture-python-intro-html-{tag}.tar.gz - Full HTML site archive
• 🔐 html-checksum.txt - SHA256 verification file
• 📋 html-manifest.json - Build metadata (tag, commit, timestamp, size, file count)

## Key Features

• ✅ Does not modify _build/html/ directory (gh-pages deployment unaffected)
• ✅ Preserves existing release notes (no body override)
• ✅ Automatic tag detection from workflow context
• ✅ Creates safety net before Phase 2 (gh-pages history cleanup)

MAINT: update anaconda=2025.06 and python=3.13 (#637)

* MAIN: Update to anaconda=2025.06

* update environment

* adjustments for compat in exercise nodes

* adjust title on eigen_I.md

* adjust fig

* tmp: disable build cache

* re-enable build cache

* update cache bevaiour to schedule or manual build

MAINT: upgrade quantecon-book-theme==0.8.2 (#577)

FIX: FutureWarning in heavy_tails (#581)

* FIX: FutureWarning in heavy_tails

* Update lectures/heavy_tails.md

* minor typo

* fix install of wbgapi

* fix cntry_mapper

[french_rev] review of FutureWarnings (#580)

Tom's edits of typo in french revolution lecture

[greek_square] Fix a minor mistake (#579)

Co-authored-by: Matt McKay <mmcky@users.noreply.github.com>

Change to `np.NAN` --> `np.nan` (#570)

This is deprecated and removed fully in numpy 2.0.

Please see:
https://numpy.org/doc/1.26/reference/constants.html#numpy.NAN

Tom's Feb 4 edits of pv.md lecture in intro series -- corrected answe…

…rs to exercise 4

Tom's edits of tax smoothing lecture, Feb 3