#70 follow-up: surface .r-version mismatch at R session startup

The CLI's `uvr sync` already refuses to wipe-and-rebuild when the
calling R's minor differs from the project pin (since v0.2.19), but a
user who *just* opens R against the project — never running sync —
wouldn't see that warning and could be running scripts under the
wrong R for ages. B-Nilson asked for a session-startup check.

Adds a small block to the .uvr-managed `.Rprofile` snippet that:
- reads `.r-version` if present
- compares its minor to `getRversion()`'s minor
- emits a one-line `message()` on mismatch with actionable next steps

Existing projects pick up the new check on the next `uvr init` /
`uvr sync` via `refresh_uvr_block`, which rewrites the marked block
in-place without touching user-added .Rprofile content.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: nbafrank

crates/uvr/src/commands/init.rs

@@ -123,10 +123,31 @@ const RPROFILE_SNIPPET: &str = r#"# >>> uvr >>>
 local({
   lib <- file.path(getwd(), ".uvr", "library")
   lock <- file.path(getwd(), "uvr.lock")
+  rver_file <- file.path(getwd(), ".r-version")
   count_locked <- function(path) {
     if (!file.exists(path)) return(0L)
     length(grep("^\\[\\[package\\]\\]", readLines(path, warn = FALSE)))
   }
+  # #70 follow-up: warn when the active R session minor doesn't match
+  # the project's `.r-version` pin. The CLI's `uvr sync` already refuses
+  # to wipe-and-rebuild on a minor mismatch since v0.2.19, but a user
+  # who *just* opens R against the project (no sync yet) wouldn't see
+  # that warning and could be running scripts under the wrong R for
+  # ages. Surface it at session startup so the mismatch is visible
+  # immediately.
+  if (file.exists(rver_file)) {
+    pinned <- tryCatch(trimws(readLines(rver_file, warn = FALSE)[1]),
+                       error = function(e) "")
+    if (nzchar(pinned)) {
+      active <- as.character(getRversion())
+      pinned_minor <- paste(strsplit(pinned, "\\.")[[1]][1:2], collapse = ".")
+      active_minor <- paste(strsplit(active, "\\.")[[1]][1:2], collapse = ".")
+      if (pinned_minor != active_minor) {
+        message("uvr: R ", active, " active but .r-version pins ", pinned,
+                ". Restart R against the pinned version, or run uvr::r_pin() to change the pin.")
+      }
+    }
+  }
   if (dir.exists(lib)) {
     .libPaths(lib)
     n_locked <- count_locked(lock)