fix: support today in date arithmetic

[konard]

Summary

• recognize today as a dynamic date expression
• resolve it to the current calendar date in the configured local timezone (UTC by default)
• support date arithmetic such as today - 17.01.2023
• add regression coverage for the reported expression and UTC/local date boundaries

Reproduction

Before this change, today - 17.01.2023 failed with Parse error: Unexpected identifier: today.

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features
• node scripts/check-file-size.mjs
• node --test scripts/*.test.mjs
• cargo test --all-features --verbose
• cargo test --doc --verbose

Fixes #199

[konard]

Working session summary

Implemented and finalized PR #200.

• Added timezone-aware today support.
• Fixed today - 17.01.2023.
• Added regression and timezone-boundary tests.
• Added a changelog fragment.
• Pushed commit 3c6a7eb0.
• Marked PR ready for review.
• Fresh CI passed all lint, test, package, WASM, and web-build checks.
• Working tree is clean; PR merge state is clean.

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[konard]

🤖 Solution Draft Log

This log file contains the complete execution trace of the AI solution draft process.

💰 Cost estimation:

• Model: GPT-5.6 Sol
• Provider: OpenAI
• Public pricing estimate: $11.414508

📊 Context and tokens usage:

• 200.0K / 200K (100%) input tokens, 13.3K / 128K (10%) output tokens

Total: (200.0K + 8.8M cached) input tokens, 13.3K output tokens, $11.414508 cost

🤖 Models used:

• Tool: OpenAI Codex
• Requested: gpt-5.6-sol
• Thinking level: off (disabled)
• Model: GPT-5.6 Sol (gpt-5.6-sol)

📎 Log file uploaded as Gist (2644KB)

• View complete solution draft log

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Now working session is ended, feel free to review and add any feedback on the solution draft.

[konard]

✅ Ready to merge

This pull request is now ready to be merged:

• All CI checks have passed
• No merge conflicts
• No pending changes

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