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, the same change as mach-json#4.

• mach.toml: [project] uses work in place of out, and the artifact out sits under {project.work}. [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 6bf8512.

Decisions

• mach = "^6" matches std dev. It was ^6.10.
• The profiles match std dev's exactly: optimize, debug and simd only. vectorize = false in debug was dropped, since optimize = false runs no passes. vectorize = true in release was dropped, since the default set under optimize = true already runs it. float_reassoc = false was dropped from both, since exact semantics is the default.
• [artifact.<id>] gains export = true. On mach dev a bare use <id>; binds only the library artifact marked export, and default = true no longer implies it.
• ci.yml is mach-json's re-seeded workflow verbatim, since the package workflows were identical before.

Ran

mach dep pull ., mach build . --all, mach test . --all --timeout 10m and mach fmt --check . with the mach dev compiler, all passing on linux-x86_64. The other targets were built, not run.