fix(history): align rollups to completed wall-clock minutes

[albertovincenzi]

Problem

The counter task slept for a fixed 60 seconds from process startup and stamped the resulting delta with the current minute. A process started at 12:00:37 therefore measured 12:00:37 through 12:01:37 but stored the whole sample as 12:01:00. Charts and rate calculations received shifted buckets containing parts of two real minutes.

Fix

• calculate the delay to the next UTC minute boundary on every pass
• store the counter delta under the minute that just completed
• add boundary tests for an exact boundary, one millisecond after it, and one millisecond before the next one

Dependency

This PR is intentionally based on #13. The reliable checkpoint from that PR must land first so a failed history write cannot advance the sample baseline. Once #13 merges, GitHub can merge this PR as the small follow-up commit shown here.

Review notes for Alice

The exact-boundary case deliberately waits a full minute. Flushing immediately there would create a zero-length sample and move the following delta into the wrong row. The timestamp is also deliberately one minute behind the wake-up boundary because it names the interval that ended at that boundary.

Verification

• cargo test -p gate-server --lib
• cargo clippy -p gate-server --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

[alice-viola]

Landed on master via #67 (merge commit 944ee9b) as part of the 62-PR integration — this PR's head commit d55da1d is an ancestor of master. GitHub could not mark it merged automatically because its base is fix/durable-rollup-checkpoints, not master. Closing as landed.