Hidden series participate in x/y-domain inference and always emit an `[extends-*]` warning

Context

The series-to-reference envelope kind needs a way to reference a bound that isn’t drawn as a visible series (e.g. the equality diagonal in a Lorenz curve). Two patterns considered:

1. Hidden series — declare a normal series with hidden: true, reference it by id in the envelope. Series participates in compute but not in rendering.
2. Envelope kind that skips the series layer entirely — already delivered as series-to-reference referencing a declared references entry, which has no rendered body by default if referencesStyle.show is false.

We ship both — option 2 is the canonical single-source-of-truth for Lorenz; option 1 exists for scenarios where the hidden bound is not a straight line.

Decision

hidden: true semantics locked as:

Concern	Behaviour
Point validation	Runs normally — [series.dropped-points] fires
X-domain inference	Participates — [hidden.extends-x-domain] always fires when widened
Y-domain inference	Participates — [hidden.extends-y-domain] always fires when widened
Palette allocation	Skipped — visible series still get palette[0..] densely
Series rendering	Suppressed — no path, markers, or hover
Trendline	Not computed even if trendline: true
End label + legend	Excluded
meta.totalSeries	Counted
meta.dataSeries	Counted (has ≥1 valid point)
meta.visibleSeries	Not counted
highlightSeries → hidden id	Ignored + [highlight.hidden-target] warning
Envelope targetability	Yes — the canonical pattern for series-pair

The [hidden.extends-*-domain] warning always fires when a hidden series widens the domain — no suppression flag. Rationale: hidden series referenced by one envelope can silently squash another. Over-warning is cheaper than silent data loss. Consumers who expect the extension filter by warning code — the [code] bracket prefix is designed for exactly this.

Consequences

• All-hidden charts fall through to empty-state (rather than rendering axes with no data). The empty-state gate checks visibleSeriesCount, not dataSeriesCount.
• meta.accessibleLabel counts visibleSeries — screen readers hear “Line chart, 2 series” not “Line chart, 4 series (2 hidden)”.
• Palette allocation uses a visible-only counter: if series b is hidden between a and c, then c gets palette[1] (not palette[2]).