refactor: explicit public exports, elevation* naming, deprecations

[kenjinp]

Summary

@hello-terrain/three re-exported the whole quadtree/ and tasks/ barrels, leaking the LOD driver internals; a few names still used heightmap where the spec says elevation.

• index.ts now uses explicit named exports. Removed from the public surface: update, createState, beginUpdate, allocLeafSet, resetLeafSet, allocSeamTable, resetSeamTable, buildSeams2to1, SeamTable, buildLeafIndex, createSpatialIndex, U32_EMPTY, runTileBoundsReduction. The topology contract, factories, task refs, and graph data types stay public.
• Dead code deleted: NodeFlags, isLive, NodeStore.flags, NodeStore.roots; createNodeStore(maxNodes) no longer takes spaceCount.
• Renames with @deprecated aliases: sampleHeightmapMeters → sampleElevationTextureMeters, HeightmapTexture → ElevationTexture (module tsl/heightmap.ts → tsl/elevationTexture.ts), createTerrainFieldTextureTask → createTerrainFieldStorageTask.
• Renamed outright (object keys, no alias): terrainTasks.createTerrainFieldTexture → createTerrainFieldStorage, terrainTasks.createTileNodes → tileNodes.
• createComputePipelineTasks(leaf, { name }) derives ${name}CompileTask / ${name}ExecuteTask so custom pipelines don't collide with the defaults in graph.inspect(); test added.
• elevationFn.displayName("elevationFn"); stale packHalf2x16 doc on terrainFieldStageTask rewritten for the RGBA pack.
• "sideEffects": false on three/react/work (audited for import-time side effects; none required). CJS build left as is.
• Docs: new "Elevation textures" section, createComputePipelineTasks docs, changelog; specs architecture.md, naming-conventions.md, terrain-data-model.md updated; docs components updated to new names.

Verification

• typecheck, lint (0 warnings; fixed the one pre-existing), vitest three 71/71, work 70/70, react 4/4, pnpm -r build succeeds and three's dist export list no longer contains the removed symbols.
• Bugbot adversarial review (leftover imports across react/docs/benchmarks, TerrainTasks type vs keys, sideEffects safety, alias correctness): no findings. Grep across packages/react, apps/docs, READMEs finds no stale references.

Merge notes

• Conflicts expected with fix: never skip midPipelineExecute; harden compute pipeline #24 (nodeStore.ts: flags removal vs allocNodeRaw) and docs: align README/docs with the real API #25 (both add a compute-stages section to elevation-function.mdx; keep docs: align README/docs with the real API #25's version of the example, which builds the graph without the default compute tasks). Changelog "Unreleased" overlaps with all open PRs.
• Beans: hello-terrain-b5k2.

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