ShellSyntaxTree 0.3.0-alpha.5

This prerelease adds an explicit host-selected PowerShell dialect contract and
keeps Bash and PowerShell analysis at the native host boundary. The v0.2
projection remains supported, and the public API changes are additive.

Added

• Add PwshDialect and PwshParserOptions.Dialect, defaulting to the
  compatible PowerShell 7 behavior and failing closed for unknown enum values.
• Model Windows PowerShell 5.1 as an explicit native-Windows fallback with its
  own alias catalog, receiver conservatism, and rejection of unsupported
  && / || pipeline-chain syntax.
• Carry the selected dialect through nested PowerShell parsing and select the
  matching dialect for static same-language pwsh and powershell.exe child
  hosts.

Security and compatibility

• Keep shell languages separate: Bash treats pwsh as an ordinary external
  command, and PowerShell treats bash as an ordinary external command. The
  parser never interprets a child command string in the other language.
• Require consumers to select PowerShell7 only for a compatible PowerShell
  7.6 host (>=7.6.4 and <7.7) and use WindowsPowerShell51 for the native
  powershell.exe fallback. Host selection, parser dialect, approval policy,
  and executor identity must agree.
• Preserve the v0.2 compatibility projection and report exactly two additive
  public API changes: the dialect enum and the parser-options property.
• Expand the generated PowerShell corpus to 504 entries. Linux and Windows CI
  validate it with hash-pinned PowerShell 7.6.4; Windows additionally validates
  the Windows PowerShell 5.1 dialect with the native executable.