Discover Bash heredoc substitutions

[Aaronontheweb]

Summary

• discover supported command substitutions inside expanding Bash heredoc bodies
• preserve literal quoted or escaped delimiters, exact authored spans, tab stripping, and isolated substitution state
• fail closed for unsupported header tails, queued heredocs, backticks, arithmetic, continuations, malformed bodies, and depth overflow
• add full structural corpus expectations, real-Bash oracles, and a reusable Bash corpus authoring check

Validation

• Release build: clean, zero warnings
• full tests: 1,424 passed
• header verification: passed
• OpenSpec strict validation: passed
• Slopwatch on changed C# files: zero findings
• package creation: passed
• adversarial review: GO on staged diff SHA-256 ffc22305587c432dccfff49cb898d3271870b4793f1400cfcff9323611f5cfb9