P1: File tailer max open files + idle eviction #193

Problem

tail.rs HashMap of open files grows unbounded. No eviction. Deleted files leak FDs forever (last_read field exists but is never checked for eviction).

K8s node with 1000 pods = potentially 1000+ open FDs with no limit.

Fix

1. Add max_open_files: usize to TailConfig (default: 1024)
2. After the read loop in poll(), add LRU eviction: if files.len() > max_open_files, sort by last_read, evict oldest until under limit
3. Add cleanup for deleted files: if !path.exists() during poll, remove from files HashMap
4. Track open_files metric

Test scenarios

1. Create 10 files, set max_open_files: 5. Assert only 5 FDs open, 5 least-recently-read evicted.
2. Write to an evicted file → re-opened on next glob rescan.
3. Delete a file → FD cleaned up on next poll.
4. open_files metric tracks count accurately.

Files to change

• crates/logfwd-core/src/tail.rs: add config field, eviction pass after line 404, deleted file cleanup
• crates/logfwd-config/src/lib.rs: add max_open_files to input config