docs: document the xazz run --opt (IR optimization) flag in README

[ax1s-x1zz]

Summary

xazz run --opt enables the typed-IR optimization pass (e.g. filter reordering) in the execution engine, but it is not documented anywhere in README.md or docs/. Users can discover it only via xazz run --help.

Context

• The flag was added in the CLI: src/cli.rs (Run.opt), threaded through src/main.rs → xazz-runner → xazz-exec (--opt/--optimize).
• The pass lives in xazz-compiler/src/opt.rs (optimize_program) and is equivalence-tested in xazz-exec/src/lower.rs (optimizer_preserves_filter_select_reorder_semantics).
• README's command table (README.md ~line 213) lists xazz capabilities; the run subcommand docs show --output/--json but not --opt.

Task

1. Document --opt in README.md under the CLI usage / examples section with a one-line explanation (typed-IR pass, preserves semantics) and an example.
2. Add a matching line to README_kr.md (the Korean mirror) so both stay in sync.
3. (Optional) mention it in docs/ARCHITECTURE.md where the IR optimizer is described.

Definition of Done

• README.md and README_kr.md both mention xazz run --opt and what it does.
• No behavior change; cargo test --workspace still passes.
• Keep user-facing text English-first (README_kr is the Korean mirror; that is fine as it is the documented Korean translation of the README).

Notes

• Follow the existing README formatting conventions (tables, code blocks).
• This is a documentation-only change — the right size for a first PR.