Extract shared native argument fragment classification

Problem

PR #68 added equivalent native-argument fragment walks to the Bash and PowerShell parsers. Both implementations now collect contiguous lexer fragments, preserve the complete raw span and decoded value, detect opaque fragments, and safe-fail resolver-sensitive mixed quoting. Keeping those rules duplicated creates a drift risk when another fragment shape or shell is added.

This is not a request for a shared lexer or parser core. Bash and PowerShell must retain their shell-specific tokenization, quoting, recursion, and command semantics.

Proposed scope

Extract an internal shell-neutral classification result for one native argument assembled from parser-owned lexer fragments. Each parser adapts its own tokens into the helper and remains responsible for consuming the resulting token range.

The shared result should carry:

• the complete authored source span and raw value;
• the combined decoded logical value;
• whether any fragment is opaque or computed;
• whether all value fragments are literal;
• whether mixed literal and expandable fragments contain resolver-sensitive syntax;
• the first index after the consumed fragment run.

Non-goals

• Unifying the Bash and PowerShell lexers.
• Building a shared parser base class.
• Changing the public AST or parser APIs.
• Weakening DynamicSkip behavior for ambiguous mixed quoting.

Acceptance criteria

• Bash and PowerShell use one internal fragment-classification implementation.
• Shell-specific token adapters remain small and explicit.
• Existing Raw, Value, source-span, path, and DynamicSkip behavior remains unchanged.
• The PR #68 adversarial corpus cases for quoted values, maximal fragment runs, unquoted prefixes, mixed literal syntax, and curl @ transformation remain green in both shells.
• dotnet build -c Release, dotnet test -c Release, and header verification pass.

Extraction trigger

Do this before adding a third shell or another native fragment rule. Until then, the current duplication is correct but carries maintenance cost.

Confronta le scansioni dei frammenti degli argomenti nativi di Bash e PowerShell introdotte da PR #68, quindi esamina i relativi casi del corpus avversariale per valori quotati, sequenze massime, sintassi mista e trasformazione di curl @. Estrai solo la classificazione shell-neutral, mantenendo separati gli adattatori dei token e il comportamento del parser. Esegui dotnet build -c Release, dotnet test -c Release e la verifica degli header; il lavoro è terminato quando tutto il comportamento esistente continua a essere verde in entrambe le shell.