docs: define time and Clock contract

[vycdev2]

Summary

• select Clock.now(view clock) -> time.Timestamp as the canonical wall-clock API
• define signed-millisecond Timestamp/Duration values, deterministic clock injection, error behavior, and removal of ambient time.now_ms/time.now_s
• document the source/runtime boundary, implementation slices, backend handoff, and align the core design, architecture, and progress references

Verification

• cargo fmt --check — passed
• cargo build — passed
• cargo test -q — passed, including all 432 driver fixtures
• git diff --check — passed

Risk

• Medium: this is documentation-only, but it selects public time semantics and an implementation direction that should be reviewed before code changes begin.

Closes #75

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.

[vycdev]

The time/Clock contract reviewed as internally sound, but this documentation branch now conflicts with current main after the earlier architecture/design/progress updates landed. Please rebase and reconcile the current canonical text rather than overwriting it, then rerun diff/link checks.