fix(snapshot): preserve exact requested times

[miguel-heygen]

What

Explicit snapshot --at timestamps now reach the timeline seek unchanged, so before/midpoint/after evidence can target exact fractional-frame transition boundaries.

Why

Snapshot rounded every requested time to three decimal places. For values such as 101 / 30, that moves the sample relative to the requested boundary and can capture the wrong side of a deterministic transition.

How

Only caller-supplied --at values retain full precision. Automatically generated default and readable-tail samples remain normalized, and existing tail de-duplication behavior is unchanged.

Test plan

• Added a regression test for an exact 3.3666666666666667s transition timestamp
• bun run --cwd packages/cli test -- snapshot.test.ts (13/13)
• bun run --cwd packages/cli typecheck
• bunx oxlint and bunx oxfmt --check on changed files
• Pre-commit hooks, including typecheck and tracked-artifact checks

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