fleet: fix parent-escape ERE double-escape + arm (T110 delta B2)

The parent-escape rejection grep used a double-escaped ERE ('\\.\\.'), which
matches a literal backslash followed by any character - never a normal '..'
component - so an absolute HOME containing '/../' traversed to a planted tree
and executed it (rev-code-02 delta probe: rc 0, no diagnostic, sentinel
present). Corrected to the single-escaped ERE ('\.\.'), which matches a
literal dot-dot component.

Suite: A8 parent-escape hostile arm added (absolute HOME
<base>/../escape-target, executable planted at the resolved tree outside the
seat home) - the real launcher must refuse with the parent-escape diagnostic
and leave the sentinel absent, and a guard-bypassed mutant copy MUST execute
the same plant under the identical arm (sentinel present), proving the check
is what stands. The arm's HOME traversal was also corrected during authoring:
the first draft double-prefixed the worktree path, which made the candidate
nonexistent and would have let the mutant pass vacuously (the same
wrong-resolved-path class B3 caught).

Marker T110-DISPATCH-TOPHER-1N2O / ORCH-T110-B2FIX-X8Y9.

packages/mosaic/framework/fleet/bin/mosaic

@@ -86,7 +86,7 @@ fallback_candidate_usable() {
             return 1
             ;;
     esac
-    if printf '%s' "$candidate" | grep -qE '(^|/)\\.\\.(/|$)'; then
+    if printf '%s' "$candidate" | grep -qE '(^|/)\.\.(/|$)'; then
         echo "mosaic: refusing \$HOME candidate $candidate: parent-escape component" >&2
         return 1
     fi

packages/mosaic/framework/fleet/bin/test-mosaic-launcher.sh

@@ -159,4 +159,35 @@ set -e
 [ "$rmrc" = "0" ] || fail "A7b mutant did not execute the relative plant (rc $rmrc, out '$rmout') - A7 proves nothing"
 [ -e "$WORK/relative-sentinel" ] || fail "A7b mutant ran but relative sentinel absent - arm wrong, A7 proves nothing"
 
+# A8 — parent-escape hostile arm (rev-code-02 delta, B2 remains): an absolute
+# HOME containing a literal '..' component must be refused by the
+# parent-escape check — the traversal would otherwise land on a planted tree
+# OUTSIDE the seat home with no symlink involved.
+ESC_BASE="$WORK/escape-base"
+ESC_TARGET="$WORK/escape-target"
+mkdir -p "$ESC_BASE" "$ESC_TARGET/.npm-global/bin"
+cat >"$ESC_TARGET/.npm-global/bin/mosaic" <<SH
+#!/bin/sh
+touch "$WORK/escape-sentinel"
+echo "0.0.0-escape-planted"
+SH
+chmod +x "$ESC_TARGET/.npm-global/bin/mosaic"
+set +e
+eout="$(printf '' | env GETENT_STUB=fail HOME="$ESC_BASE/../escape-target" PATH="$STUB_BIN:/usr/bin:/bin" "$LAUNCHER" --version 2>&1)"
+erc=$?
+set -e
+[ "$erc" = "127" ] || fail "A8 parent-escape HOME was followed (rc $erc, out '$eout')"
+case "$eout" in *"parent-escape component"*) ;; *) fail "A8 parent-escape diagnostic missing: '$eout'" ;; esac
+[ ! -e "$WORK/escape-sentinel" ] || fail "A8 escape plant EXECUTED"
+
+# A8b — mutation control: the guard-bypassed copy MUST execute the parent-
+# escape plant under the identical arm (sentinel present, rc 0), proving the
+# parent-escape check is what stands.
+set +e
+emout="$(printf '' | env GETENT_STUB=fail HOME="$ESC_BASE/../escape-target" PATH="$STUB_BIN:/usr/bin:/bin" "$MUTANT" --version 2>&1)"
+emrc=$?
+set -e
+[ "$emrc" = "0" ] || fail "A8b mutant did not execute the escape plant (rc $emrc, out '$emout') - A8 proves nothing"
+[ -e "$WORK/escape-sentinel" ] || fail "A8b mutant ran but escape sentinel absent - arm wrong, A8 proves nothing"
+
 echo "mosaic launcher suite: all arms passed"