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.

Cargo.toml

@@ -1,8 +1,8 @@
 [package]
 name = "libfreemkv"
-version = "0.19.0"
+version = "0.19.1"
 edition = "2024"
-rust-version = "0.19.0"
+rust-version = "1.86"
 license = "AGPL-3.0-only"
 description = "Open source raw disc access library for optical drives"
 repository = "https://github.com/freemkv/libfreemkv"
@@ -10,7 +10,7 @@ keywords = ["bluray", "uhd", "optical", "scsi", "disc"]
 categories = ["hardware-support", "multimedia"]
 
 [dependencies]
-serde = { version = "0.19.0", features = ["derive"] }
+serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 sha1 = "0.10"
 sha2 = "0.10"
@@ -22,7 +22,7 @@ num-traits = "0.2"
 num-integer = "0.1"
 rand = "0.8"
 cmac = "0.7"
-zip = { version = "0.19.0", default-features = false, features = ["deflate"] }
+zip = { version = "2", default-features = false, features = ["deflate"] }
 base64 = "0.22.1"
 # Trace-level instrumentation for Disc::copy + SgIoTransport::execute. Permitted
 # under project docs ("Acceptable strings: debug/trace logging"). Consumers (autorip)