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refactor: remove internal/h3core and reorganize x/h3go

Remove the internal/h3core package and inline the shared H3 index-format
constants and pentagon base-cell table into both consumers, so neither
depends on the shared package:
- h3: bit-layout constants become literals (or C macros where the block
  already uses them); the pentagon table becomes a package-level var.
- x/h3go: same constants and pentagon table inlined, keeping the package
  fully cgo-free and self-contained.

Reorganize x/h3go alongside the inlining:
- Group const/var/type declaration blocks at the top of each source file.
- Split error vars into errors.go; rename h3go.go -> constants.go and add
  h3.go for the package doc.
- Move type declarations out of constants.go into the files where their
  methods live (projection primitives -> faceijk.go, etc.).
- Convert bboxFromGeoLoop/bboxesFromGeoPolygon to methods
  (GeoLoop.toBbox / GeoPolygon.toBboxes).
- Replace the shared Cell bit-manipulation methods with generic functions
  over the Index constraint (modeOf, reservedBits, resolution, indexDigit,
  indexDigitChecked, ownerCell) so DirectedEdge and Vertex no longer cast to
  Cell to reuse them; no DirectedEdge/Vertex -> Cell casts remain.
- Add a typed `mode` (iota: cellMode=1, directedEdgeMode=2, edgeMode=3 unused,
  vertexMode=4); modeOf returns it.

No behavior change; parity tests, 100% coverage, vet, and golangci-lint all pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>