S8: relix fmt — canonical script printing #12

Part of #17. Design: docs/design.md §3.

Scope

relix fmt [SCRIPT...] prints a script canonically, using the engine's statement printer, the one behind Relix.definitions().

• --glyphs / --keywords choose the operator spelling.
• -w rewrites files in place, as gofmt -w does.
• --check exits 1 if any file would change, for CI.

Comments must survive. If the printer drops them, this slice raises a relix-core issue before shipping rather than losing them.

Dependencies

• S1 (S1: command skeleton — script sources, exit codes, diagnostics, profiles, engine options #4).

Acceptance

• Formatting is idempotent.
• Parse → fmt → parse gives the same statements over every example script.
• --check exit codes.
• Comments are preserved.