- Replace 10 different installation methods with unified uv approach
- Update target Python version from 3.12 to 3.14
- Add sections for virtual environments and multiple Python versions
- Remove outdated options (Anaconda, Chocolatey, Homebrew, building from source)
- Reduce guide from 454 lines to ~180 lines

uv python install only creates a versioned python3.14 executable, so the
macOS and Linux verification steps told readers to run `python -V` and
expect success when they would actually hit "command not found" -- right
at the guide's "You're all set!" line. Install with --default on all three
platforms, which creates the python and python3 executables the guide
promises, and drop the `uv run python -V` workaround that was papering
over this on Windows.

Verified against uv 0.11.29: a clean install now yields python -V ->
Python 3.14.6 on all platforms.

Also:
- Correct sample output; uv prints no "Searching for Python 3.14" line
  and lists the installed executables
- Update 3.14.2 -> 3.14.6, note patch version and timing will vary, and
  resolve the "3.12+ recommended" vs installing 3.14 contradiction
- Rewrite venv activation to teach isolation rather than availability,
  since --default makes "you can use python directly" a non-statement
- Warn that --default repoints python when installing multiple versions
- Fix two silently-ignored .rumdl.toml keys: disabled-rules -> disable
  and MD048.code-fence-style -> MD048.style, which is why intentional
  inline HTML was flagged on every run
- Refresh README's stale "complex decision tree" framing

Formatting only: list markers to '-', blank lines before the horizontal
rules following the HTML anchors, blank lines after changelog headings,
and a consistent horizontal rule style.

One non-cosmetic fix: MD044's proper-names rule was rewriting the link
text "https://github.com/..." to "https://GitHub.com/...", so the page
would render a capitalized URL on every fmt run. The link text is now
descriptive rather than a raw URL, which sidesteps the rule and also
drops the dangling "visit the GitHub repository here".

Move the Unreleased entries into a dated release matching the existing
YYYY.MM.DD - slug convention, and leave Unreleased empty for next time.

Prose:
- Replace em dashes with plain hyphens or shorter sentences across the
  guide and README, keeping dashes where they earn their place
- Tighten the --default explanation, which used "default" three times
- Fix a comma splice in the experimental-warning note
- "which interpreter python points to" rather than tangling Python/python

Step 3 no longer opens by telling readers to close and reopen a terminal
they already reopened in Step 1. Once ~/.local/bin is on PATH, python
resolves as soon as the install finishes (verified with an isolated PATH),
so the reopen is now a fallback after the expected output, where someone
who actually needs it will look.

Two changelog entries were stale: one claimed we added the reopen reminder
we ended up removing, and the fmt/MD044 work was unrecorded.