feat(workout): add secondary swim CSS offset target

[fasoxy-it]

The typed workout helpers support pace, speed, cadence, power, and heart-rate targets, but swimming CSS offsets still require manually constructing the secondary target fields.

This adds SwimCssOffsetTarget(offset_seconds=...) and TargetType.SWIM_CSS_OFFSET (17, swim.css.offset). Both targeted step helpers now accept an omitted primary target, so callers can use Garmin's no.target primary with a CSS offset as the secondary target. Existing calls remain compatible.

The offset is a signed value rather than a lower/upper range: it is serialized into secondaryTargetValueOne, with secondaryTargetValueTwo=0. Non-finite offsets and primary placement are rejected. The README includes a secondary-only example; interval types and swimming stroke/rest/pool configuration are otherwise unchanged.

Validation

• 42 focused workout tests passed, including signed/zero offsets, both step helpers, finite-value validation, primary-placement rejection, unchanged untargeted output, and full swimming-workout serialization.
• Full offline suite: 463 passed, 22 integration tests deselected, 1 existing platform-specific failure. TestTokenFilePath.test_absolute_path_outside_home_still_allowed uses /var/lib/...; on macOS /var is a symlink and the token path guard rejects it. Neither the test nor the client code was changed.
• Ruff lint and format checks passed.
• Mypy passed for all 8 source files.

Maintainer review

This is a draft proposal based on a downstream serializer's existing CSS payload shape, not a new captured Garmin response or a live upload performed for this PR. Please confirm the signed-offset interpretation, units, and secondaryTargetValueTwo=0 convention against Garmin before merging. No live-account tests or private workout/account data are included.

[coderabbitai]

Important

Draft PR not reviewed

Draft PRs are not automatically reviewed by default.

• Trigger a manual review

To automatically review draft PRs, update your CodeRabbit configuration:

reviews:
  auto_review:
    drafts: true

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

---

_{Comment @coderabbitai help to get the list of available commands.}