Fix JavaScript indexed accessors

[treeform]

Summary

• Use JS-safe indexed accessors for object-backed vectors and matrices.
• Use direct nested array assignment for array-backed matrices when compiling to JavaScript.
• Add indexed accessor coverage across vector scalar types and float matrix types, plus CI rows for JS object/array storage modes.

Testing

• git diff --check
• nim r tests/tests.nim
• nim r -d:vmathObjBased tests/tests.nim
• nim r -d:vmathArrayBased tests/tests.nim
• nim js -r tests/tests.nim
• nim js -r -d:vmathObjBased tests/tests.nim
• nim js -r -d:vmathArrayBased tests/tests.nim