Release v0.2.15: soften P3M-unavailable message

The "WARN P3M binary download failed for X, falling back to source: 404"
line read like a uvr bug to users, but it's a normal happens-sometimes
situation: Posit's package manager doesn't always build every package
version against every R minor (especially older branches like 4.4).
Falling back to source compilation is the correct behavior; users only
saw the warn-level wording as alarm.

- Detail (404 message, exact URL) drops to debug-level.
- Single info-level line: "<pkg> <ver>: no P3M binary for this R minor,
  compiling from source" — explains *why* the install slows down for
  that package without sounding like an error.

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

Author: nbafrank

Cargo.lock

@@ -3,7 +3,7 @@ members = ["crates/uvr", "crates/uvr-core"]
 resolver = "2"
 
 [workspace.package]
-version = "0.2.14"
+version = "0.2.15"
 edition = "2021"
 authors = ["uvr contributors"]
 license = "MIT"

Cargo.toml

@@ -80,10 +80,18 @@ impl Downloader {
                             used_binary: is_binary,
                         }),
                         Err(e) if is_binary && fallback_url.is_some() => {
-                            // Binary download failed — fall back to source tarball
+                            // Binary download failed — fall back to source tarball.
+                            // The most common reason here is "Posit hasn't built this
+                            // version against this R minor" (especially on older R
+                            // branches), which is normal and not a uvr error. Keep the
+                            // detail at debug-level; users see a single dim INFO line
+                            // so it's clear *why* the install is taking longer.
                             let fallback = fallback_url.as_ref().unwrap();
-                            tracing::warn!(
-                                "P3M binary download failed for {pkg_name}, falling back to source: {e}"
+                            tracing::debug!(
+                                "P3M binary unavailable for {pkg_name} {pkg_version}, falling back to source: {e}"
+                            );
+                            tracing::info!(
+                                "{pkg_name} {pkg_version}: no P3M binary for this R minor, compiling from source"
                             );
                             let path = download_one(
                                 &client,