Summary

Adds rs fmt --stdin-filepath <path>, the stdin formatting mode that editor integrations (conform.nvim, none-ls, etc.) rely on — the equivalent of Prettier's --stdin-filepath. Content is read from stdin, formatted as if it were saved at <path>, and written to stdout.

Note: this flag is an interim solution for editor integration. The long-term plan is to serve editors through LSP (the route Biome's biome lsp-proxy and oxfmt's oxfmt --lsp take). Prettier itself ships no official LSP server, so --stdin-filepath remains the de-facto editor protocol for Prettier-based formatters, and editor plugins already know how to drive it.

The semantics were decided after surveying how Prettier, Biome, and oxfmt implement their equivalent flags:

• Only the flag triggers stdin mode; combining it with --write / --check / --list-different or file arguments is an error (exit 2), matching oxfmt's parser-level exclusivity rather than Prettier/Biome's silent-ignore behavior.
• The path does not need to exist on disk and is resolved against cwd; it drives parser inference, overrides matching, and sortPackageJson.
• An ignored path (ignorePatterns or default lock files) skips formatting and writes the input unchanged with exit 0 — all three surveyed tools agree on this, and it keeps editor buffers safe. .gitignore is not consulted, consistent with how explicitly passed file arguments already behave.
• stdout carries only the formatted output; diagnostics go to stderr. No parser / parse errors exit 2 with an empty stdout. Empty input produces empty output with exit 0. EPIPE from an early-closing reader (| head) is not an error.
• stdin is fully drained before any failable work surfaces, so a blocked writer sees the real error instead of EPIPE; config loading overlaps with the drain.

Implementation

• src/fmt/format.ts: formatting core extracted from worker.ts (plugins → parser inference → Prettier format), with a lazy source reader so unsupported files are still skipped before reading. worker.ts keeps only file I/O and changed/unchanged semantics.
• src/fmt/stdin.ts: the stdin runner — single file, main thread, no worker pool. Plugins load on demand through the shared fmtPlugins chunk, keeping the lazy-loading behavior introduced in perf(fmt): load plugin resolver on demand #164.
• src/fmt/cli.ts: flag parsing (kebab + camel, following the existing convention), conflict validation, and a one-line stdin branch.

Tests

• Unit: flag parsing and conflicts, formatting core (options, sortPackageJson, unsupported-before-read, parse errors).
• Integration (spawned CLI with piped stdin): byte-exact stdout for .ts/.json, overrides application, ignored-path passthrough, unknown-extension and parse-error exit codes, flag conflicts, empty input.

Full suite: 176 tests green; lint, type-check, and format check clean.