feat(interpreter): stream pipeline stages concurrently with SIGPIPE

crates/bashkit/docs/threat-model.md

@@ -114,6 +114,7 @@ through configurable limits.
 | sleep bypasses timeout (TM-DOS-057) | `sleep N` ignores `ExecutionLimits::timeout` | Host-backed timeout; non-JS wasm blocking sleep is clamped to the execution deadline | **MITIGATED** |
 | Interactive terminal flooding (TM-DOS-119) | Unbounded type-ahead, undrained output, or a `vi` buffer grown by pastes | Caps on unread input (1 MiB), retained output (4 MiB), scrollback (1000 rows), transcript (1 MiB), line length, `vi` buffer (8 MiB) and undo history; `:w` goes through VFS limits | MITIGATED |
 | Input wait vs. timeout (TM-DOS-120) | Abusing the keyboard-wait exclusion to run work past the deadline | Only time blocked reading the terminal is excluded; CPU work and `sleep` still count | MITIGATED |
+| Concurrent pipeline amplification (TM-DOS-124) | An endless loop piped into `head`, a producer whose reader quit, or recursive pipelines `f() { f \| f; }` | 4 KiB pipes with backpressure; a writer whose reader is gone ends with 141 (SIGPIPE); streaming nesting counts toward `max_subshell_depth` and falls back to sequential stages past 4 levels; stages share the session budget, timeout and cancellation | MITIGATED |
 | Random generator amplification (TM-DOS-123) | `openssl rand` asked for gigabytes | Requests capped at 1 MiB; fixed-size `uuidgen`/`$SRANDOM` | MITIGATED |
 | Unbounded builtin output (TM-DOS-058) | `seq 1 1000000` produces 1M lines | Add `max_stdout_bytes` limit | **MITIGATED** |
 | Silent truncation at builtin caps (TM-DOS-109) | `seq 200000`, an awk loop past its cap, or an oversized `sprintf` expression returns incomplete output with exit 0 | Caps report `<cmd>: <what> limit (<N>) exceeded` on stderr and exit non-zero; awk caps and formatting errors are fatal | **MITIGATED** |