Add structured Bash command substitutions

[Aaronontheweb]

Summary

• recursively discover Bash $() commands in supported simple-command arguments and redirect targets
• preserve exact syntax, command ordering, ancestry, spans, isolated cwd behavior, and v0.2 dynamic compatibility values
• fail closed on dynamic command names, legacy backticks, unsupported interiors, background lists, and assignment prefixes
• add executable Bash corpus cases 190-199 plus live Bash boundary oracles

Scope

This is the first Bash substitution slice. Iterables, expanding heredoc bodies, PowerShell, Netclaw integration, the consumer guide, and Web/Mermaid samples remain explicit follow-up work.

Verification

• Release build clean
• 1,278 tests passed
• headers and formatting verified
• strict OpenSpec validation passed
• corpus JSON and PII audit passed
• Slopwatch clean
• adversarial review GO: 85cbabbb3f54165f372f340b2d24ea1d9bfc09742e6662486d51d20910685d0c