v0.19.1: repair Cargo.toml after botched 0.19.0 bump

v0.19.0 was tagged with a search-and-replace gone wrong:
rust-version, serde, and zip all had their version strings
replaced with "0.19.0". Edition 2024 rejected rust-version
0.19.0 (< 1.85), failing every CI build. No artifacts shipped
to crates.io.

Repair:
- rust-version: 0.19.0 → 1.86 (CI pin)
- serde: 0.19.0 → 1
- zip: 0.19.0 → 2

Also drops an unused start_lba binding in mux/disc.rs that
clippy 1.86 catches.

src/mux/disc.rs

@@ -318,14 +318,13 @@ impl DiscStream {
         // on failure, advance on success. One 5s read attempt per try — no
         // retry loops, no sleeps. On size-1 failure, skip or error.
         //
-      // Halt is checked at the top of every iteration — in a dense bad zone
+        // Halt is checked at the top of every iteration — in a dense bad zone
         // this loop can spend minutes shrinking and skipping sectors; without
         // the check, Stop wouldn't take effect until the outer PES read() loop
         // finally emits a frame, which may never happen.
-        
-        let start_lba = lba;
+
         let start_time = std::time::Instant::now();
-        
+
         loop {
             if self.is_halted() {
                 return Err(crate::error::Error::Halted.into());