fix(ci): track Cargo.lock and pin lopdf for MSRV 1.75 compatibility (#15

)

## Summary

- Remove `Cargo.lock` from `.gitignore` for reproducible CI builds with
`--locked` flag
- Pin `lopdf` to 0.36 (lopdf 0.37+ requires Rust 1.85/edition2024,
incompatible with MSRV 1.75)
- Ensures CI passes on both MSRV (1.75) and latest stable Rust

## Changes

1. **`.gitignore`**: Removed `Cargo.lock` entry to track lockfile
2. **`Cargo.toml`**: Changed `lopdf = "0.38"` → `lopdf = "0.36"` with
explanatory comment
3. **`Cargo.lock`**: Generated and committed for reproducible builds

## Test Plan

- [ ] CI passes on all platforms (ubuntu, macos, windows)
- [ ] MSRV check (Rust 1.75) passes
- [ ] `--locked` builds work correctly

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Len P. van der Hof <Lenvanderhof@ReasonKit.sh>

ReasonKit Core v0.1.4

chore(release): bump versions to 0.1.2

Publishing new versions to crates.io with latest fixes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>