internal.log_lines: preserve microsecond timestamp order across writer batches

[jshearer]

Description:

The log writer gives every line in a batch the same timestamp, so timestamp-based pagination can skip the rest of a batch. Assign each line a timestamp at PostgreSQL's microsecond precision, at least one microsecond beyond the previous line and no earlier than the current clock reading. Retain the previous timestamp across batches and backward clock adjustments.

Deployment:

This PR is stacked on #3545. Deploy this writer fix before or with the discovery API; when landing the stack in order, deploy after both PRs have landed. #3553 has no dependency on this stack.

Validation:

The real-writer pagination regression passes in the API, agent, and models suite and in the combined run with current master and #3553. This commit builds with the offline SQL cache.

[strix-security]

Strix Security Review

No security issues found.

Review summary

Reviewed all three files in PR #3550. The production change in crates/control-plane-api/src/logs.rs assigns each internal.log_lines row a monotonic, microsecond-precision logged_at timestamp derived solely from the server clock and the previous batch's timestamp, and binds it as the fourth parameter of the existing insert ... unnest($1,$2,$3,$4) statement. The query remains fully parameterized across token, stream, log_line, and logged_at, so no attacker-controlled data reaches a dangerous sink. The new next_logged_at helper operates only on server-generated clock values, and its panic path is unreachable for realistic timestamps. The remaining two files are test-only: a #[cfg(test)]-gated integration test refactored to exercise the real writer path, and its corresponding snapshot updated to redact clock-derived values. No injection, authentication/authorization, secret-handling, cryptography, deserialization, or other security-relevant change was identified.

_{Updated for b3dbc25.}

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Reviewed by Strix
_{Re-run review · Configure security review settings}

[bbartman]

LGTM.