perf: enable thin LTO in release profile

[persimmon16]

Summary

• Adds lto = "thin" to the workspace release profile for better cross-crate optimization
• Thin LTO provides most of the binary size and performance benefits of full LTO with significantly faster link times

Related Issue

Production readiness audit finding (P3): release profile has strip = true but no LTO.

Changes

• Cargo.toml (workspace root): Added lto = "thin" to [profile.release]

Testing

• cargo check passes
• Thin LTO is a well-established Rust optimization that requires no code changes

Checklist

• Conventional commit format
• No secrets committed
• Scoped to the issue at hand

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