[octalide]

Closes #567

What

--format now defaults to the host system's object format. The host is the mink system row for $mach.build.sys (linux, darwin, windows), and the default is the first format of its formats list (default first) that names the architecture. All of assemble, listing and inline go through context.open_output and object.format_for, so one path covers them. An explicit --format is unchanged and wins. The ambiguous refusal is replaced by hostless: the host's formats name no code for the architecture.

Decisions

• The host is picked by $mach.build.sys, as mach-std does, and only selects mink's row. No masc format table. A new host system is one line in object.host, a new format needs no CLI edit.
• A cross arch takes the host's first format that names it. Where none does and the architecture has exactly one format (wasm, spirv), that one is taken, as before. Only when the host names none and several formats do is it refused as hostless.
• A host mink has no row for (a BSD) gives no host system, so masc still compiles there and falls to the single-format rule or hostless.

Notes

• mink's catalog has no BSD system row, so a BSD host needs --format where several formats name the architecture, until mink and object.host state one.
• No new tests, per the implementation-only rule. check . -a '*' and fmt . are clean.