feat(orm): add standard date and time value codecs

[GaspardKirira]

Summary

Add explicit standard-library date and time conversions for ORM fields.

Current behavior

The mapper compatibility layer does not directly support std::chrono values.

Applications must convert dates and timestamps to strings or integers manually.

Expected behavior

The ORM should provide documented codecs for common std::chrono representations.

Storage format must be explicit and stable.

Scope

• Support a timestamp representation.
• Support date-only values where practical.
• Define UTC handling.
• Define precision.
• Define text or integer storage formats.
• Support optional temporal values.
• Add SQLite integration tests.
• Add MySQL integration tests where available.
• Document timezone limitations.

Suggested files

• modules/orm/include/vix/orm/codecs/Chrono.hpp
• modules/orm/tests/chrono_codec_test.cpp
• modules/orm/docs/...

Acceptance criteria

• Timestamp encoding is deterministic.
• Timestamp decoding preserves documented precision.
• UTC expectations are documented.
• Empty optional timestamps map to NULL.
• SQLite round-trip tests pass.
• Existing mapper APIs remain compatible.

Non-goals

This issue does not include:

• timezone database management;
• locale-specific formatting;
• automatic schema migrations;
• calendar arithmetic.

Suggested labels

• scope:orm
• type:feature
• tests
• help wanted
• status:accepted