chore: bump MSRV to 1.78 for Cargo.lock v4 compatibility

Cargo.lock version 4 was introduced in Rust 1.78. The lock file in this
repo uses v4, which caused CI MSRV check to fail with Rust 1.75.

Changes:
- Update rust-version in Cargo.toml from 1.75 to 1.78
- Update CI workflow MSRV job to use rust-toolchain@1.78.0
- Update README badges and requirements

Author: decebal

.github/workflows/ci.yml

@@ -131,7 +131,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@1.75.0
+      - uses: dtolnay/rust-toolchain@1.78.0
       - uses: Swatinem/rust-cache@v2
         with:
           workspaces: "apps/core -> target"

README.md

@@ -170,7 +170,7 @@ allsource-core = "0.2"  # Pin to minor version for stability
 ## Development
 
 ### Prerequisites
-- **Rust**: 1.75+
+- **Rust**: 1.78+
 - **Go**: 1.21+
 - **Elixir**: 1.19+ (with Erlang/OTP 27+)
 - **Bun**: 1.1+ (for TypeScript/web apps)

apps/core/Cargo.toml

@@ -2,7 +2,7 @@
 name = "allsource-core"
 version = "0.7.3"
 edition = "2021"
-rust-version = "1.75"
+rust-version = "1.78"
 authors = ["AllSource Team"]
 description = "High-performance event store core built in Rust"
 license = "MIT"

apps/core/README.md

@@ -4,7 +4,7 @@
 
 [![crates.io](https://img.shields.io/crates/v/allsource-core.svg)](https://crates.io/crates/allsource-core)
 [![docs.rs](https://docs.rs/allsource-core/badge.svg)](https://docs.rs/allsource-core)
-[![Rust](https://img.shields.io/badge/rust-1.75%2B-orange.svg)](https://www.rust-lang.org/)
+[![Rust](https://img.shields.io/badge/rust-1.78%2B-orange.svg)](https://www.rust-lang.org/)
 [![Tests](https://img.shields.io/badge/tests-250%20passing-brightgreen.svg)]()
 [![Performance](https://img.shields.io/badge/throughput-469K%20events%2Fsec-blue.svg)]()
 [![Architecture](https://img.shields.io/badge/clean%20architecture-Phase%203%20Infrastructure-success.svg)]()