Switch starter.mojo kernels and solve from fn to def

[kunal-mansukhani]

Summary

Mojo 26.2 deprecates the `fn` keyword per the RFC "Remove fn from Mojo, def no longer implies raises" (accepted 23 Feb 2026). Going forward:

• `def foo(...)` — non-raising (was `fn`)
• `def foo(...) raises` — raising form (was `fn ... raises`, replaces the old implicitly-raising `def`)

`fn` will be removed before 1.0. Every starter currently emits a deprecation warning on compile. This sweeps all 79 `starter.mojo` files to the new form now so learners don't see the dead keyword.

Mapping

before	after
`fn vector_add_kernel(...)`	`def vector_add_kernel(...)`
`@export fn solve(...) raises:`	`@export def solve(...) raises:`

Kernels drop to plain `def` — they don't raise. `solve` keeps the `raises` because it calls `DeviceContext`, `compile_function`, `enqueue_function`, and `synchronize`, all of which raise.

Test plan

• Transformed `vector_add` starter compiles cleanly on Mojo 26.2 via the prod compiler container (`mojo build --emit=shared-lib --target-accelerator=sm_75` → exit 0, exported `solve` symbol in the `.so`).
• CI lint / pre-commit passes.

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