refactor: remove unused tilemap decoding state

[joeykchen]

Remove the old-map conversion that only modified range copies, and stop expanding atlas coordinates that tile placement never reads. Keep all JSON input fields and the five-integer record format. The private expanded record shrinks from 20 to 12 bytes.

A second cleanup removes unused serialization tags and temporary variables and reads only complete records. Placement order, paths, incomplete tails, missing sources, and the existing source sentinel behavior stay unchanged.

Validation: callback-trace and old-format path tests passed before the refactor; all 78 host packages, pure-engine tilemap tests/root compilation, and WebAssembly root compilation passed with Go 1.26.5. Runtime metadata was regenerated twice with identical final diffs.

Integration note: other pending runtime refactors also update the generated export.types. Regenerate it with make generate-runtime when combining those branches. This change has no source-file overlap with refactor PRs #1975–#2001.

Combined validation of all five independent cleanup branches also passed: all host packages, codegen and interpreter submodules, pure/wasm root compilation, and actual Web binding wasm tests. Two full generation runs were identical, with all six external Godot outputs unchanged.

[fennoai]

Reviewed the complete fixed diff for the legacy tilemap loader, five-integer tile-record parsing, incomplete-tail handling, source grouping, decorator loading, world-bound calculation, and generated export metadata. I found no concrete, actionable bug introduced by this patch. Verification: go test ./internal/tilemap passes.