feat(pivot): roll-forward preferred-label bindings

[jfrench9]

Summary

Implement the deferred half of preferred-label support: roll-forward bindings. periodStartLabel / periodEndLabel roles aren't just label text — the same concept appears under two presentation arcs (beginning balance at the top of the network, ending balance at the bottom), and the beginning row must bind the instant fact at each duration column's start date (dated the day before the start, per the XBRL/SEC convention). Previously the tree dedup silently dropped the second arc, opening balances were pruned as sparse noise columns, and equity statements showed only ending balances.

No converter or SEC-adapter changes needed — both already deliver the roles and strings (xbrl-holon#8, report-components#24); this is pivot-only.

Changes

• src/pivot.ts
  • PresentationTree.childrenOf now carries arcs (child + label + binding + negated) instead of bare child ids; arcRef classifies periodStart* → 'start', periodEnd* → 'end'.
  • The walk dedups per (element, binding) so a roll-forward concept emits one row per arc, in arc order (beginning at the top, ending at the bottom); all other element repeats still drop.
  • Cell and member-combo lookups are binding-aware: 'start' rows read the instant at dayBefore(column.startDate) (exact start date as fallback); 'end'/default rows read the column end as before. A start/end row with no bindable fact anywhere doesn't render.
  • Standalone opening-instant columns are suppressed in roll-forward sections — their data now renders as the beginning-balance row inside the duration columns.
  • Row keys carry a rf:start/rf:end variant suffix so React keys and cell selection stay unique.
  • The 0.3.2 first-arc-wins label map is subsumed by the arc-carrying tree (also fixes labels when one concept has different labels under different arcs).

Testing

• npm run test:all — green, exit 0 (109 tests across 13 files; the full prior suite passes unchanged through the tree refactor).
• New test/rollforward.test.ts: beginning/ending rows for the same concept in arc order, opening-instant binding (start − 1 day) into the duration column with the fact identity asserted, standalone opening-column suppression, and row-key uniqueness.
• Real-data verification on the NVDA 10-K equity statement (bundled sample holon): beginning balances 22,101 / 42,978 / 79,327 chain into ending 42,978 / 79,327 / 157,293 across FY24–FY26 — each year's ending equals the next year's beginning — with equity-component sub-rows, negated tax-withholding rows, and the beginning/ending share-count rows, matching the as-filed statement. Verified visually in the viewer dev server against the same sample.

Notes

• SEC mode inherits the feature automatically: the graph's Association.preferred_label roles flow through the 0.3.3 adapter into the same pivot path.
• No sample regen needed downstream — the bundled holon already carries the roles; a viewer dependency bump lights this up.