Error on duplicate and colliding command registrations

[johncarmack1984]

Closes #115.

What changed

• export now fails with a clear message when a command is registered more than once, instead of silently generating a bindings object where the last registration wins
• the check runs on the cased accessor name, so it also catches two differently named commands that collide after function_casing is applied (e.g. my_cmd and myCmd both exporting as myCmd), with a message that names both commands and the colliding accessor
• regression tests for both cases plus a passing control

Why

commands.myCmd silently binding to whichever registration came last is a correctness trap — the error surfaces it at export time with the exact names involved.

Notes

Two same-named commands in different modules of one crate currently can't even be constructed — the generated __specta__fn__*/__cmd__* helper macros collide at crate root (the same family as #142) — so that scenario is only reachable across crates (e.g. a plugin command clashing with an app one), where this check still catches it. Noted in the test file.

Validation

• cargo test --all-features (3 new tests)
• cargo clippy --all-features --all-targets (no warnings)
• cargo fmt --check