Add ngr_rspm_* functions for faster GitHub Actions builds

[NewGraphEnvironment]

Problem

GitHub Actions builds with renv can take 35+ minutes when RSPM (Posit Public Package Manager) doesn't have binaries for packages in the lockfile. This happens when lockfile contains bleeding-edge package versions that RSPM hasn't synced yet.

Compiling R packages from source on Linux is slow. RSPM provides pre-built binaries that install in seconds.

Proposed Solution

Add ngr_rspm_* functions to check lockfile compatibility with RSPM and optionally sync:

# Check which packages in lockfile don't match RSPM available versions
ngr_rspm_check <- function(lockfile = "renv.lock") {
 # Returns data.frame of mismatches: package, lock_version, rspm_version
}

# Downgrade mismatched packages to RSPM-compatible versions
ngr_rspm_sync <- function(mismatches) {
 # Installs RSPM versions and updates lockfile
}

Use Case

• Run ngr_rspm_check() in CI before build
• If mismatches found, either warn or auto-sync
• Trade-off: slightly older packages vs 30+ min faster builds

TODO

• Confirm RSPM-compatible builds are significantly faster (testing in mybookdown-template)
• Design function API
• Implement ngr_rspm_check()
• Implement ngr_rspm_sync()
• Add to CI workflow templates

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Relates to NewGraphEnvironment/mybookdown-template#78
Relates to NewGraphEnvironment/sred-2025-2026#3