interp: add stmt, subshell, builtin, func-call, and var-lookup handlers

[saracen]

ExecHandlers already lets external code wrap external command execution. Apply the same middleware-chain pattern to five more dispatch points so the interpreter can be extended from outside without patching its internals.

StmtHandlers fires around every syntax.Stmt before dispatch. SubshellHandlers fires around each subshell creation site, with a SubshellKind to distinguish background, paren, cmd subst, proc subst, and pipeline. BuiltinHandler registers a name-keyed override for a builtin; BuiltinHandlers wraps all builtin dispatch with middleware. FuncCallHandlers wraps the body of a declared shell function. LookupVarHandlers wraps lookupVar on top of the runner's own special-variable and environment resolution, so middlewares can add or override variables without losing the existing behaviour.

Each chain follows ExecHandlers' shape: middlewares are chained first-to-last, the bottom of the chain is the runner's own behaviour, and a middleware may short-circuit by not calling next. Chain bottoms recover the active runner from ctx, so the chains are built once in Reset and inherited by subshell copies the same way execHandler is.

Also add RunStmts, a top-level helper for dispatching stmts on the runner attached to ctx, useful for handlers that need to execute shell code in the runner's current scope (e.g. a trap body from a Go-side signal handler).