Restore the beta.19 verb-slot rule of command words - #241

Fixes #240

What changed

• Restore the 0.4.0-beta.19 verb-slot rule of CommandOccurrence.CommandWords. ShellCommandWordProjection.cs is identical to 0.4.0-beta.19. In the verb slot, a plain word directly after an option stays a command word. After the verb slot, an option value is skipped as before.
• Reason (owner decision): without per-program grammar, a value-less flag followed by a subcommand cannot be told apart from an option value. In 0.4.0-beta.20, dropping the word let short grants cover hidden programs: git --no-pager push gave git, xargs -0 rm gave xargs, and env -i rm gave env.
• Prepare 0.4.0-beta.21 (version, release notes, README, SPEC, consumer guide, plan). SPEC and the guide return to the beta.19 text, with a short note on the revert.

Tests

• The beta.20 inputs stay. Their expectations change to the beta.19 results, with the reason in a comment (Bash_verb_slot_keeps_the_word_after_an_option, PowerShell_verb_slot_keeps_the_word_after_an_option, Bash_exact_option_binding_keeps_the_next_word_in_the_verb_slot).
• New pins (Verb_slot_word_after_a_switch_stays_visible): docker --debug run → docker run, docker --debug run img → docker run img, ilspycmd -t X /p/x.dll → ilspycmd X, git --no-pager push → git push, xargs -0 rm → xargs rm, env -i rm → env rm.
• The tests that beta.20 changed return to their beta.19 expectations (owner table, Bash_verb_slot_keeps_a_plain_word_after_an_option, hiding attempts, PowerShell position rule and words, exact binding words, the seeded option-order table, and the launch-value test). The extra inputs that beta.20 added stay, for example r=push; git -C . "$r" and git -C . "$SUB".
• New: git log --pretty=oneline main → git log main. A manual mutant that ignores an inline option value survived the full suite on beta.19 code. This test kills it.

Evidence

• dotnet test -c Release: 4621 passed, 0 failed. Build: 0 warnings. Header check passes. Pack gives ShellSyntaxTree.0.4.0-beta.21.nupkg.
• Output comparison: the same 6,057 inputs and 109,960 records as for beta.20. The beta.21 dump is byte-identical to the beta.19 dump in both launch modes. Against beta.20, only CommandWords changes, in the same 186 inputs.
• Manual mutants (each built and run): 4 killed, 0 survived. They cover the beta.20 rule, no option-value skip, option values skipped only in the slot, and an ignored inline value.
• Public API: no change.