improve precision of @env.now() on native backend

[Guest0x0]

The API @env.now() should return the duration of time since the Unix epoch in milliseconds. However, previously its implementation on native backend only has second precision. With moonbitlang/moonbit-native-runtime@eac2ffc, the latest MoonBit native runtime now provides an API for getting millisecond precision time. This PR utilizes the new runtime API to fix the precision of @env.now().

---

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.

[coveralls]

Pull Request Test Coverage Report for Build 3717

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.006%

---

Totals
Change from base Build 3714:	0.0%
Covered Lines:	14610
Relevant Lines:	15378

---

💛 - Coveralls

[bobzhang]

[AI Rally - Reviewer]

Clean, well-motivated change that improves @env.now() precision on the native backend from second-level to millisecond-level. The old implementation called C's time() (second precision) and multiplied by 1000, which meant sub-second time differences were lost. The new implementation directly calls moonbit_get_ms_since_epoch from the updated native runtime, which provides true millisecond precision. The change reduces 7 lines to 1, maintains the same now_internal() -> UInt64 signature so callers are unaffected, and the dependency on the new runtime API is documented in the PR description with a link to the runtime commit.