Add pkg_detail RPC for packages pane detail editor (#1291)

Adds a `.ps.rpc.pkg_detail(name)` RPC that returns cheap, kernel-local detail
metadata for a single installed package. Positron's new package detail editor
calls this when a package is opened.

The RPC reads the installed package's `DESCRIPTION` via
`utils::packageDescription(...)` and returns a named list with:

- `name`
- `dependencyCount` - count of `Depends` + `Imports` + `LinkingTo`, excluding
  base packages and `R` itself
- `title` - the `Title` field (when present)
- `author` - `Maintainer`, falling back to `Author` (when present)
- `license` - the `License` field (when present)
- `sourceRepository` - the `Repository` field, e.g. "CRAN" (when present)
- `publishedDate` - the `Date/Publication` field (when present)

It returns `NULL` for an empty name or a package that isn't installed. No Rust
change is needed - the generic `.ps.rpc.<method>` dispatch in
`crates/ark/src/ui/ui_comm.rs` already routes it.

Companion Positron work: **posit-dev/positron#12926** (the package detail
editor, which adds the `getPackageDetail` package-manager hook that calls this
RPC). The Positron PR will bump the ark submodule to this change once merged.

Positron PR: posit-dev/positron#14440

## QA

In an R session:

```r
.ps.rpc.pkg_detail("utils")   # base pkg: name/title/author/license, dependencyCount 0
.ps.rpc.pkg_detail("dplyr")   # CRAN pkg: + sourceRepository, publishedDate, dependencyCount > 0
.ps.rpc.pkg_detail("nope")    # NULL
```

Author: bricestacey

crates/ark/src/modules/positron/packages_pane.R

@@ -115,6 +115,75 @@
     as.list(version)
 }
 
+# Return detail fields for a single installed package by name.
+#' @export
+.ps.rpc.pkg_detail <- function(name) {
+    if (!nzchar(name)) {
+        return(NULL)
+    }
+    fields <- c(
+        "Title",
+        "Author",
+        "Maintainer",
+        "License",
+        "Repository",
+        "Date/Publication"
+    )
+    # Read straight from the package's DESCRIPTION. packageDescription() returns
+    # a length-1 NA (and warns) when the package isn't installed, which is far
+    # cheaper than installed.packages() -- that parses every installed package's
+    # DESCRIPTION just to answer an existence check.
+    d <- suppressWarnings(utils::packageDescription(name, fields = fields))
+    if (!inherits(d, "packageDescription")) {
+        return(NULL)
+    }
+
+    # Collapse DCF whitespace; return NULL for missing (NA) fields.
+    clean <- function(x) {
+        if (is.null(x) || is.na(x)) {
+            return(NULL)
+        }
+        trimws(gsub("\\s+", " ", x, perl = TRUE))
+    }
+
+    # Reduce an R License field to its primary license: the first alternative
+    # (before "|"), without the "+ file LICENSE" clause.
+    primary_license <- function(x) {
+        if (is.null(x)) {
+            return(NULL)
+        }
+        first <- trimws(strsplit(x, "\\|")[[1]][1])
+        first <- trimws(sub(
+            "\\s*\\+\\s*file\\s+.*$",
+            "",
+            first,
+            ignore.case = TRUE
+        ))
+        if (!nzchar(first)) {
+            return(NULL)
+        }
+        first
+    }
+
+    # Prefer Maintainer for author display; fall back to Author.
+    author <- clean(d$Maintainer)
+    if (is.null(author)) {
+        author <- clean(d$Author)
+    }
+
+    # Drop absent (NULL) fields so the other side only sees populated keys.
+    out <- list(
+        name = name,
+        title = clean(d$Title),
+        author = author,
+        license = primary_license(clean(d$License)),
+        sourceRepository = clean(d$Repository),
+        publishedDate = clean(d[["Date/Publication"]])
+    )
+    Filter(Negate(is.null), out)
+}
+
+
 # Return the list of outdated packages with their latest available versions.
 #
 # `utils::old.packages()` queries the user's configured repositories, so