shell: restore variant-aware runtime switches

[ewhauser]

Summary

• add a resolved shell-variant profile layer as the single source of truth for parser language, diagnostics, runtime defaults, Bash namespace exposure, and audited builtin availability
• drive runtime behavior from the resolved ShellVariant across core, interpreter, lookup, quoting, trap rendering, xtrace, and parse/reparse helpers
• gate Bash-only namespace state and audited builtin families by shell variant, update SPEC.md, and add focused runtime/profile coverage

Why

The recent multishell plumbing restored parser-level variant selection, but several runtime helpers still hardcoded Bash behavior. That left gbash with inconsistent shell-specific behavior across parsing, lookup, special vars, and builtin surfaces.

Impact

• sh now starts with POSIX mode on and braceexpand off
• mksh and zsh now consistently use their variant parser/diagnostic paths through runtime helpers
• Bash-only namespace vars and caller are hidden outside bash/bats
• compgen, complete, compopt, mapfile, readarray, and shopt are only exposed for bash/bats
• dirs, pushd, and popd are only exposed for bash/zsh
• type, help, enable -a, completion lookup, and virtual command stubs no longer advertise unsupported builtins as active
• one stale regex conformance xfail was removed after the new coverage showed gbash now matches bash there

Root Cause

Variant resolution had been reintroduced at the shell-selection layer, but downstream runtime code still fell back to Bash defaults in a number of paths. Builtin lookup and Bash namespace exposure were also not being filtered through one authoritative runtime profile.

Validation

• go test ./internal/shellvariantprofile ./internal/shell/... ./internal/runtime ./shell/expand/... -count=1
• go test ./internal/builtins -run 'TestHelp|TestBashRuntime' -count=1
• go test ./internal/conformance -run 'TestConformance/bash/oils/(builtin-bash|builtin-dirs|builtin-type|regex)\.test\.sh' -count=1 -timeout=20m
• make lint

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