feat!: say the next step on every line, and refuse -n with --all

[iwamot]

Everything a result needs said in words — a list the registry has nothing to put in, a window that dropped every version, a list cut by the limit — now prints on stderr ahead of the table. stdout carries the table alone, so a caller can read the rows with awk, and a package with nothing to list no longer answers with a bare header. With --json the cut count stays a field in the document and is not repeated on stderr.

-n and --all asked for two different lengths and the last one typed won, while the same pair next to @VERSION was already refused. Both orders now exit 2.

An unknown flag, a transport failure, and a status that is neither the document nor a 404 carry a remediation like the rest of the lines: a rate limit says how long to wait on every registry, a 5xx says the registry is failing and to retry later, and any other status says the registry is refusing this request. RateLimited moved to internal/fetch so PyPI and npm reach the same wording.

The instruction paragraph no longer reads as if @VERSION combines with the options that narrow a list, and says that stderr explains an empty or cut table on exit 0. A test keeps the README copy of the paragraph verbatim.

🤖 Generated with Claude Code

[github-actions]

Dependency Review

✅ No vulnerabilities found.

Scanned Files

None

[codecov]

Codecov Report

❌ Patch coverage is 65.90909% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.58%. Comparing base (2650cfb) to head (fd7c3a4).

Files with missing lines	Patch %	Lines
main.go	52.38%	10 Missing ⚠️
internal/fetch/fetch.go	85.71%	2 Missing ⚠️
internal/pypi/fetch.go	0.00%	2 Missing ⚠️
internal/npm/fetch.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
+ Coverage   69.65%   70.58%   +0.93%     
==========================================
  Files          11       11              
  Lines         491      510      +19     
==========================================
+ Hits          342      360      +18     
- Misses        149      150       +1     

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