[spike] Tabbed SelectPanel on the experimental composable SelectPanel

[joshfarrant]

Draft — not for merge. Part of a spike comparing how a tabbed SelectPanel (Branches / Tags, one shared search) gets built across Primer's APIs. Same task, same model (Opus 4.8); the API is the only variable:

• [spike] Tabbed SelectPanel on the stable SelectPanel (faked tabs) #7969 — stable SelectPanel, pragmatic (tabs work visually but aren't ARIA-associated)
• [spike] Tabbed SelectPanel with correct ARIA on the stable API (forced rebuild) #7970 — stable SelectPanel, ARIA correctness forced (the component gets abandoned)
• [spike] Tabbed SelectPanel on the experimental composable SelectPanel #7967 — experimental composable SelectPanel (composes correctly, ~35 lines of glue)
• [spike] Modular SelectPanel with tabs (layered) #7966 — proposed 4-layer modular SelectPanel (composes correctly, zero glue)

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Builds the same picker on the experimental composable SelectPanel (@primer/react/experimental). The list lives inside the tab panel, so the tab→tabpanel→listbox relationship is real. ~165 lines, of which ~35 are glue that exists only because the layering is incomplete: hand-rolled Tab/TabList/TabPanel wrappers, a ref type cast, manual per-tab counts, and the API being filed under "experimental/deprecated".

See packages/react/src/SelectPanel/examples/RefPicker.tsx. This is the "correct, but with glue" middle of the comparison.

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[primer]

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