feat(detector): implement N (tj/n) detection

[dipto0321]

Summary

Add the N (tj/n) detector — the 6th of 8 Node.js version managers in the Phase 1 detection rollout.

Unlike the other Unix-binary managers (asdf/mise), n is a bash script with no rich metadata — it just echoes its script-internal VERSION constant and dumps directory paths from find. The implementation is intentionally minimal.

Changes

• internal/detector/n.go (full implementation):
  • Detect(): platform.LookupManagerBinary("n") != ""
  • Version(): n --version, returns the script's VERSION constant verbatim (no v prefix; e.g., 10.2.0). Both --version and -V work because the upstream script handles both (bin/n line 1694).
  • ListInstalled(): n ls, parses node/<version> lines. The upstream regex "/[0-9]+\.[0-9]+\.[0-9]+" filters non-semver entries before we see them.
  • Mutation methods return ErrNNotImplemented sentinel (matches the ASDF/Mise pattern)
• internal/detector/n_test.go (24 mocked tests):
  • parseNVersion: standard output (10.2.0), v prefix, leading/trailing whitespace, empty, whitespace-only
  • parseNInstalled: real n ls output, unsorted input → sorted, empty stdout, blank lines, malformed lines (no slash, trailing slash), non-semver versions, deeper paths (forward-compat for node/v20/20.11.1)
  • Method tests: Name, Version (success/V-prefixed/runShell error/parsing error), ListInstalled (success/empty/runShell error/skips unparseable)
  • Detect: PATH-without-binary, PATH-with-binary on the stub (Windows-safe via n.exe)
  • MutationMethods_NotImplemented sentinel test

Design notes

• n is Unix-only per its README (explicitly says no PowerShell/Git Bash/Cygwin support). On Windows the detector returns false because platform.LookupManagerBinary("n") won't find n.exe in PATH. Users on Windows must use NVMWindows/Nodenv/Volta — nodeup will silently skip n for them.
• No $N_PREFIX check in Detect(): unlike asdf which accepts the data dir as a positive signal, n requires its $N_PREFIX/bin/n script to be on PATH to be runnable. A data dir alone is not actionable. This is a deliberate asymmetry — documented in the code comments.
• LastIndex for path parsing: n ls outputs node/<v> lines today, but the parser uses strings.LastIndex(line, "/") so a hypothetical future node/v20/20.11.1 layout would Just Work without code changes.
• Pre-sorted upstream: n's own sort -k 1,1 -k 2,2n -k 3,3n -k 4,4n -t . produces numeric-by-semver ordering. We re-sort defensively in case upstream changes the algorithm.

Test plan

• go test ./... — all packages green
• go vet ./... — clean
• gofmt -l — clean
• CI matrix: lint (ubuntu), test on ubuntu/macos/windows, build on 5 platforms — pending

[cocogitto-bot]

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