Abstract the hamburger / icon-stroke geometry into a --ds-* token family

DEC-0046 added an accessible mobile disclosure to the shared SiteHeader. Its two-bar→X hamburger glyph is the only value in the component still drawn with bare pixels — height: 2px on the bars (.site-menu-toggle__glyph, ::before, ::after) plus the ±6px vertical offsets and the translateY(±6px) morph. Everything else in the component is an existing --ds-* token, so this is the lone token-discipline bypass left after the polish pass.

Why it matters

Establish the design system and the UX-engineering function makes the typed token tree the single source of truth and styles/dossier.css a drift-tested generated cache — bare px in a component is exactly the bypass that discipline exists to prevent. @dossier/design has no --ds-border-width-* / --ds-icon-* family yet (confirmed: grep of packages/design finds neither), so this is a small additive token family, not a refactor. Adding it makes the glyph geometry tunable and auditable like every other dimension, and gives the next icon/stroke need a token to reach for. Owned by Principal UX Engineer (the design-language function, DEC-0010).

Notes

Board globbed before filing — no open task covers an icon/border-width token family (the design-adjacent task-checked-in-contrast-utility is a WCAG checker and task-prune-dead-per-page-chrome-css is Astro dead-code, both unrelated). Provenance: surfaced by the log-auditor recording DEC-0046, where the Principal Forward Deployed Engineer flagged it as deliberately out of scope for the polish pass; confidence: inferred (agent-judged real follow-up, not human-curated). The component ships correct today — this is a hygiene/discipline close-out, hence p2.