fix(orientation): defensive coerce in hashSeed to handle undefined objID

[rulkens]

Summary

Fixes the intermittent runtime crash on hover:

fallbackOrientation.ts:33 Uncaught (in promise) TypeError:
  Cannot mix BigInt and other types, use explicit conversions
    at hashSeed (fallbackOrientation.ts:33:24)
    at fallbackOrientation (fallbackOrientation.ts:56:26)
    at buildPointInfo (pointInfoBuilder.ts:267:14)
    at pointInfoFromGlobal (engine.ts:431:12)
    at setHovered (engine.ts:478:37)

Root cause

pointInfoBuilder calls fallbackOrientation(cloud.objIDs[idx]!, ra, dec). During a tier swap, the GPU pick texture can briefly return a global index that out-runs the just-uploaded cloud buffers — idx is out-of-bounds, BigUint64Array[oob] returns undefined, the ! non-null assertion is a TS-only lie, and undefined & 0xffffffffn throws.

Fix

Defensive coerce objID to 0n at hashSeed entry when it isn't a bigint. The resulting orientation stays deterministic (derived from RA/Dec, which is always valid) and produces exactly the same value an unmatched row would — so the user sees a stable orientation that matches the rest of the catalogue's "objID = 0n" path documented in the module header.

The deeper engine-side race (pick-texture index can briefly desync from the freshly-uploaded cloud buffer) is a separate concern. This PR keeps the renderer crash-free while that's investigated.

Test plan

• npm run typecheck clean
• New regression test covers fallbackOrientation(undefined as any, ra, dec) — passes
• All existing fallbackOrientation tests pass
• Manual: reproduce a tier swap (small → large), hover during the load — no console error

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