fix(pwndbg): prevent core-triggered executable discovery

Occupy GDB's executable slot with its owned /proc/self/exe while loading a
core, then discard it before mapping queries. Select the validated decoder
architecture explicitly and restrict build-ID lookup to the owned empty path.
Preserve cancellation reasons at debugger admission.

Keep the original fixture executable present during syscall verification and
add Debian 13 / GDB 16 coverage alongside Ubuntu 24.04 in the CI definition.

Validation: controlled producer regressions and request-profile controls;
real GDB 16.3 / strace on Linux arm64 shows the old bridge opening the
recorded executable and the fix returning the same mapping without that
access. A cross-architecture gdb-multiarch probe confirms profile selection.
The plugin was controlled; full native pwndbg and Linux x64 remain unverified.
No performance improvement claimed; task tokens/CPU/family memory unmeasured.

Fixes #4914

.github/workflows/real-recorded-crash.yml

@@ -24,7 +24,19 @@ jobs:
   real-recorded-crash:
     runs-on: ubuntu-24.04
     timeout-minutes: 15
+    strategy:
+      fail-fast: false
+      matrix:
+        image: ["ubuntu:24.04", "debian:trixie-slim"]
+    container: ${{ matrix.image }}
     steps:
+      - name: Install fixture and debugger prerequisites in the disposable container
+        shell: bash
+        run: |
+          apt-get update -qq
+          apt-get install --yes --no-install-recommends \
+            ca-certificates git python3-venv gcc libc6-dev strace gdb
+
       - uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # v4.4.0
       - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4.4.0
         with:
@@ -36,11 +48,6 @@ jobs:
       - uses: ./.github/actions/portable-build-cache
         with:
           build_cache_artifact: ${{ inputs.build_cache_artifact }}
-      - name: Install GDB on the disposable CI runner
-        shell: bash
-        run: |
-          sudo apt-get update -qq
-          sudo apt-get install --yes --no-install-recommends gdb
       - name: Acquire exact unchanged upstreams in isolated runner storage
         shell: bash
         run: |