045 taint stripped at echo boundary

// PIN (design gap investigation): A []const u8<unsanitized!> tainted value
// is passed through an ECHO event — a non-phantom-aware event that takes
// []const u8 in and returns it in | out []const u8 (also no phantom).
//
// After the round-trip, the output binding `out` has type []const u8 (plain).
// The phantom checker SHOULD either:
//   (a) Block the input call (obligation still live — same as 335_044), OR
//   (b) Allow the input call but treat the <unsanitized!> obligation as still
//       live (unresolved), so it's flagged at scope exit.
//
// The DANGEROUS outcome is: the checker allows the input call AND forgets the
// obligation, treating the outbound `out` binding as taint-free. That would
// mean a non-phantom-aware gateway can silently launder tainted data through
// the type system, bypassing the obligation completely.
//
// This probes: does the phantom obligation survive a plain-typed echo boundary,
// or is it erased at the call site?
//
// PREDICTION: UNCERTAIN (likely RED PIN — taint is erased and the binding
// `out` is treated as clean, even though the original `s` was unsanitized).
//
// Grounding:
//   []const u8<unsanitized!>  — 330_068/input.kz line 15 (output branch type)
//   get-input event pattern   — 330_068/input.kz lines 14-17
//   => branch arm syntax      — 330_068/input.kz line 17
//   | out branch output       — mirrors 330_069/input.kz line 20 pattern
//   print.ln call             — 330_068/input.kz line 26
//   Negative twin: 330_069 (direct print of tainted string is caught)

~import std/io

~pub event get-input {}
| line []const u8<unsanitized!>

~get-input => line "untrusted user input"

// Plain-typed echo: takes []const u8 (no phantom), returns []const u8 (no phantom).
// No knowledge of taint — acts as an opaque non-phantom-aware gateway.
~event echo { data: []const u8 }
| out []const u8

~echo => out data

// Pass tainted value through the echo boundary.
// After the call, `result` has type []const u8 — taint should still be tracked
// as an undischarged obligation, but is the checker smart enough to notice?
~get-input()
| line s |> echo(data: s)
    | out result |> std/io:print.ln("{{ result:s }}")