fix: support bare years in datetime subtraction

[konard]

Summary

• interpret an unadorned four-digit year as January 1 when subtracting it from a datetime
• support the same coercion when the bare year is the left operand
• preserve ordinary numeric subtraction and reject out-of-range year coercion

Reproduction

Before this change, (now - 2023) in months failed with Cannot subtract number from datetime.

Verification

• added tests/issue_201_bare_year_datetime_tests.rs covering the reported expression, both operand orders, equivalent explicit January 1 dates, numeric arithmetic, and an invalid year
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features
• node scripts/check-file-size.mjs
• cargo test --all-features
• cargo test --doc --all-features

Fixes #201

[konard]

Working session summary

Implemented and finalized PR #202.

• (now - 2023) in months now works.
• Bare four-digit years resolve to January 1 during datetime subtraction.
• Both operand orders are supported.
• Normal numeric arithmetic remains unchanged.
• Added regression tests and a changelog fragment.
• Local formatting, Clippy, file-size checks, full tests, and doc tests pass.
• Fresh CI passed all applicable Rust, WASM, package, and web checks.
• PR is clean, mergeable, and ready for review.

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This summary was automatically extracted from the AI working session output.

[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: $9.309731

📊 Context and tokens usage:

• 162.9K / 200K (81%) input tokens, 13.9K / 128K (11%) output tokens

Total: (162.9K + 7.1M cached) input tokens, 13.9K output tokens, $9.309731 cost

🤖 Models used:

• Tool: OpenAI Codex
• Requested: gpt-5.6-sol
• Thinking level: low (~7999 tokens)
• Model: GPT-5.6 Sol (gpt-5.6-sol)

📎 Log file uploaded as Gist (2189KB)

• 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]

🎉 Auto-merged

This pull request has been automatically merged by hive-mind.

• All CI checks have passed

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Auto-merged by hive-mind with --auto-merge flag