refactor: trim docs and include retry count in poll message

- Remove Debian/Ubuntu install guide and RUSTFLAGS example from
  CONTRIBUTING.md (audience can figure these out)
- Extract retry count to a variable and include it in the FUSE
  mount poll panic message

https://claude.ai/code/session_01LfpnUZrgq93MVZgA3KVqE6

Author: claude

CONTRIBUTING.md

@@ -369,17 +369,7 @@ Some integration tests require external (non-Cargo) tools that are **not** manag
 - `squashfuse` (provides `squashfuse`) — cross-device (`--one-file-system`) FUSE test
 - `fuse3` (provides `fusermount3`, `/dev/fuse`) — cross-device (`--one-file-system`) FUSE test
 
-On Debian/Ubuntu:
-
-```sh
-sudo apt install squashfs-tools squashfuse fuse3
-```
-
-Tests that need these tools will panic with a diagnostic message if they are missing. The panic message includes the specific `RUSTFLAGS='--cfg pdu_test_skip_*'` flag to skip the test. For example:
-
-```sh
-export RUSTFLAGS='--cfg pdu_test_skip_cross_device'
-```
+Tests that need these tools will panic with a diagnostic message if they are missing. The panic message includes the specific `RUSTFLAGS='--cfg pdu_test_skip_*'` flag to skip the test.
 
 ## Automated Checks
 

tests/one_file_system.rs

@@ -224,15 +224,16 @@ fn cross_device_excludes_mount() {
 
     // Wait for the FUSE mount to become readable (exponential backoff)
     let wait_ms_base = 100;
-    let poll_result = (0..5)
+    let retries = 5;
+    let poll_result = (0..retries)
         .map(|exponent| wait_ms_base << exponent)
         .map(Duration::from_millis)
         .map(sleep)
         .filter_map(|()| mount_point.read_dir().ok())
         .find_map(|mut entry| entry.next());
     assert!(
         poll_result.is_some(),
-        "FUSE mount at {mount_point:?} not ready after retries"
+        "FUSE mount at {mount_point:?} not ready after {retries} retries"
     );
 
     let build_expected_tree = |one_file_system: bool| -> String {