Drop pre-2.35 CmdStan support in cmdstanr v1.0

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Some of the code was developed with the help of codex. I also had it take a look at the code changes and generate this summary:

In cmdstanr v1.0 we will only support CmdStan >= 2.35.0. Users needing older CmdStan versions can install an older cmdstanr release. This simplifies legacy compatibility code and tightens behavior around version/path handling.

Main changes:

• Require CmdStan >= 2.35.0.
• Enforce this minimum in install_cmdstan() for:
  • explicit version,
  • release_url/release_file when the archive name is parseable,
  • and the unpacked archive itself.
• Reject unsupported existing installations in set_cmdstan_path() and
  cmdstanr_initialize().

Windows/toolchain updates:

• Deprecate CMDSTANR_USE_MSYS_TOOLCHAIN and ignore it (with warning).
• Remove legacy MSYS/pacman/Rtools35 code paths.
• Add ABI-aware Rtools fallback resolution.
• Add/extend Windows CI diagnostics for toolchain candidate selection.
• In CI debug checks, use cmdstanr:::is_rtools4x_toolchain_usable() for consistency
  with package logic.

WSL/path state handling:

• Reset cached .cmdstanr$WSL state when path/version state is explicitly cleared.
• Scope cmdstan_default_path(dir=...) to the supplied directory (do not fall back to
  unrelated WSL CmdStan installs when an explicit dir is provided).

Compatibility/docs/tests/CI:

• Bump minimum R version to >= 4.0.0.
• Remove obsolete pre-R-3.5 tempdir branches.
• Update NEWS + install docs (and regenerate Rd).
• Add/extend tests for version parsing/rejection, env-var init behavior, Windows
  fallback behavior, and WSL state reset behavior.
• Keep a pinned windows-2022 (R 4.1) CI job for legacy toolchain coverage; skip
  posterior::pareto_smooth-dependent init tests only on that configuration.
• Harden recursive make test subprocess env (R_HOME/R_LIBS/R_LIBS_USER) and Makefile quoting.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Jonah Gabry

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.15385% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.92%. Comparing base (f429f12) to head (9c579c3).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
R/install.R	86.41%	11 Missing ⚠️
R/path.R	83.33%	5 Missing ⚠️
R/zzz.R	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1144      +/-   ##
==========================================
+ Coverage   87.95%   89.92%   +1.96%     
==========================================
  Files          14       14              
  Lines        5935     5913      -22     
==========================================
+ Hits         5220     5317      +97     
+ Misses        715      596     -119     

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[jgabry]

@SteveBronder @WardBrian If we require CmdStan 2.35 in cmdstanr v1.0 then I think we can simplify a lot of the Windows toolchain stuff. But I don't fully trust myself with Windows, so would be great to get your feedback if you have time. (You don't necessarily need to look at every line of code if you don't have time, but at least at a high level do these changes make sense?).