docs: document the release profile for WASM builds

[Yerimahjr]

Summary

Closes #143. Documents the [profile.release] settings used for WASM builds in the README, with a brief note on what each one does to size or performance.

Changes

• Added a "Release profile" subsection under "## Building" in README.md.
• Table covers all 8 settings from the root Cargo.toml's [profile.release]: opt-level, lto, codegen-units, strip, debug, debug-assertions, overflow-checks, panic — each with a one-line explanation of its effect.
• Values in the table match Cargo.toml exactly.

Scope

Documentation-only change in README.md. No code, no Cargo.toml, no tests touched.

Test results

• cargo fmt --check — clean
• cargo clippy --all-targets -- -D warnings — clean
• cargo test — 105 passed, 0 failed, 0 ignored

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