refactor: install_tinytex() calls install-bin-unix.sh and install-bin-windows.ps1 directly

[Copilot]

Summary

This PR refactors install_tinytex() so that the binary install path delegates to the official installer scripts (install-bin-unix.sh and install-bin-windows.ps1, downloaded from https://tinytex.yihui.org/) instead of re-implementing the same logic in R. The source install path (via install_tinytex_source()) already used install-unx.sh and is unchanged.

Changes

tools/install-bin-unix.sh

• Added TINYTEX_PREVENT_INSTALL env var check (abort if set to true)
• Added TINYTEX_TEXDIR env var: lets callers specify the full installation path directly (e.g., the R package passes this for custom dir= installs); falls back to the traditional TINYTEX_DIR-based default. For prebuilt bundles, the extracted archive is renamed to the custom path if needed.
• Added ./tlmgr conf texmf max_print_line 10000 post-install (matches post_install_config() in R, fixes log line-wrapping — issue Automatically install hyphenation patterns when using polyglossia #322)
• Fixed $1/--no-path argument conflict: $1 was used to save the installer to an output dir (for build scripts), but R passes --no-path as $1. Now checks that $1 doesn't start with -- before treating it as a save directory.
• Quoted $(dirname "$TEXDIR") to handle paths with spaces correctly.

tools/install-bin-windows.ps1

• Added TINYTEX_PREVENT_INSTALL env var check
• Added --no-path argument support (skips & $tlmgr path add when passed)
• Added TINYTEX_TEXDIR env var for full-path installs (with move+rename support for custom leaf names)
• Added & $tlmgr conf texmf max_print_line 10000 post-install
• Fixed $args[0]/--no-path conflict for the installer save-path feature
• Removed the double rd $env:TINYTEX_DIR\TinyTeX (now handled cleanly via $TargetDir)

R/install.R

• Added install_via_script(): downloads and runs the appropriate OS installer script, then does R-specific post-install steps (r_texmf(), repo config, extra_packages). Sets env vars TINYTEX_INSTALLER, TINYTEX_VERSION, and (for custom dirs) TINYTEX_TEXDIR before calling the script.
• install_tinytex() now routes binary installs through install_via_script() instead of install_prebuilt().
• install_prebuilt() is kept intact for any direct callers (it is mentioned in NEWS.md as accessible via tinytex:::install_prebuilt()).

Differences handled outside scripts (cannot be eliminated)

The following install_tinytex() behaviors remain in R and are not replicated in the scripts:

Feature	Reason
r_texmf()	R-specific: registers R's texmf tree with TinyTeX
extra_packages	Requires R package discovery (tl_pkgs())
tlgpg installation for HTTPS repos	Complex R logic with Windows/macOS check
Repository accessibility check + fallback to CTAN	R's tlmgr() integration
Interactive prompts for force / existing LaTeX	Requires readline() / interactive()
TEXINPUT env var warning	Informational, R-only
version='latest' → GitHub release lookup	Requires xfun::github_releases()
Bundle→extra_packages mapping for source installs	R-only (source install path unchanged)

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