chore: build on mach dev and track std dev

[octalide]

Moves the package onto the mach dev compiler and pins std to its dev branch.

• mach.toml: [project] uses work in place of out, and mach = "^6". The artifact out sits under {project.work}. Profile opt, vectorize and float_reassoc become optimize and the default pass set, so both profiles match std dev's exactly. [dep.std] uses ref = "branch/dev" in place of version.
• ci.yml: the install step is replaced by the "build mach from its dev branch" step from mach-std's dev CI. Nothing else in the workflow changed.
• dep/std: moved to mach-std dev at 3b525fa.

Decisions

• mach = "^6" matches std dev. It was ^6.10.
• Debug profile vectorize = false was dropped, since optimize = false runs no passes, matching std dev.
• Release profile vectorize = true was dropped, since the default set under optimize = true already runs it.
• float_reassoc = false was dropped from both profiles, since exact semantics is the default.

Ran

mach dep pull ., mach build . --all, mach test . --all --timeout 10m and mach fmt --check . with the mach dev compiler (6.10.1), all passing on linux-x86_64.