Document and enforce that just recipe arguments are positional

[ciaranra]

just --list prints each recipe as a signature (build profile="debug"), which reads like call syntax. Typing just build profile=release therefore passes the literal string profile=release as the positional argument.

Verified behavior against just 1.45.0 with a scratch justfile:

command	result
just say release	word=[release]
just say word=release	word=[word=release] (positional, not an override)
just word=release say	error: Variable \word` overridden on the command line but not present in justfile`
just greeting=yo say	greeting=[yo] (real override, before the recipe name)
just say greeting=yo	word=[greeting=yo] (even a real variable name is consumed positionally)

Documentation

• Three-line note in the default recipe output, immediately above the recipe list where the signatures appear.
• New "Passing arguments to a recipe" section in docs/development/dev-tools.md, next to its existing just build release example.

Diagnostic fixes

• python-ci-core and python-ci-smoke delegated profile validation to their build dependency, so a bad profile recommended the dependency's name (just python-ci-build-test release) instead of the recipe the user invoked. Both now validate directly.
• bench checked each slot only against its own parameter name, so just bench release pattern=foo bound pattern=foo to features and reached cargo as --features pattern=foo. Each slot now rejects every parameter name.

All cases exercised:

$ just bench release pattern=foo
Invalid features argument: pattern=foo
Just recipe parameters are positional. Use: just bench release '' 'foo'

$ just bench release '' features=simd
Invalid pattern argument: features=simd
Just recipe parameters are positional. Use: just bench release 'simd'

$ just python-ci-core profile=release
Invalid profile argument: profile=release
Just recipe parameters are positional. Use: just python-ci-core release

Not changed: just julia-build rustflags="-C ..." puts rustflags=... in the profile slot, which already fails loud with Unknown profile: rustflags=-C .... No silently-wrong behavior there.