fix(security): anti-replay, 0600 client key, open redirect, supply-chain

Address findings from a repo security review:

- core: add a sliding-window anti-replay filter over the AEAD-authenticated
  sequence in Session (poll_input/poll_frame), closing the input-replay gap the
  data plane previously left to the LAN/VPN trust assumption. 4096-deep window,
  unit-tested; the encrypted loopback suite confirms no false drops.
- clients: write the mTLS client private key 0600 and lock the config dir 0700
  on Unix (it was world-readable at the umask default), re-locking existing
  stores on load. pf-client-core::trust plus the probe's own identity writer.
  Windows keeps the %APPDATA% ACL; Android/Apple already wrap the key.
- web: fix a post-login open redirect — resolve `next` via URL and require it to
  stay same-origin, rejecting `/\evil.com` and tab/encoding variants the old
  `!startsWith("//")` guard missed. Also fixes the dead safeNextPath helper.
- ci: SHA-256-pin the BtbN FFmpeg DLLs bundled into the signed Windows installer
  (were fetched from the rolling `latest` tag unverified); fails closed on a
  re-roll, matching the VB-CABLE gate.
- ci: fail-open fork-guard on the Windows/Apple host-mode PR build jobs that
  share runner labels with the signing jobs. Definitive fix stays server-side
  (Gitea outside-collaborator approval / isolated PR runners) — see the notes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/apple.yml

@@ -16,8 +16,17 @@ on:
   workflow_dispatch:
 
 jobs:
+  # SECURITY: builds/tests PULL-REQUEST code on the host-mode, persistent `macos-arm64` runner shared
+  # with the release-signing job (release.yml, which loads the App Store Connect key). Untrusted PR
+  # code could persist on it or harvest signing material. Definitive fix is server-side: enable Gitea's
+  # "require approval for PRs from outside collaborators/forks", and/or isolate PR CI on ephemeral
+  # runners. The `if:` is a fail-open backstop — it skips fork PRs where Gitea reports the fork flag and
+  # still runs same-repo PRs (and where the flag is absent), so it never blocks internal PR CI.
   swift:
     runs-on: macos-arm64
+    if: >-
+      gitea.event_name != 'pull_request' ||
+      gitea.event.pull_request.head.repo.fork != true
     timeout-minutes: 60
     steps:
       - uses: actions/checkout@v4

.gitea/workflows/windows-drivers.yml

@@ -30,8 +30,15 @@ on:
 # scripts/ci/ensure-windows-toolchain.ps1, a fast no-op once already present.
 
 jobs:
+  # SECURITY: builds PULL-REQUEST code on the host-mode, persistent `windows-amd64` runner shared with
+  # the release-signing jobs (windows-host.yml / windows-msix.yml). See windows.yml for the full
+  # rationale. Definitive fix is server-side (Gitea outside-collaborator approval + isolated PR
+  # runners); the `if:` is a fail-open backstop that never blocks internal PR CI.
   probe-and-proto:
     runs-on: windows-amd64
+    if: >-
+      github.event_name != 'pull_request' ||
+      github.event.pull_request.head.repo.fork != true
     timeout-minutes: 30
     defaults:
       run:
@@ -108,6 +115,9 @@ jobs:
   # DLL's FORCE_INTEGRITY (/INTEGRITYCHECK) bit — the M0 self-signed-load question.
   driver-build:
     runs-on: windows-amd64
+    if: >-
+      github.event_name != 'pull_request' ||
+      github.event.pull_request.head.repo.fork != true
     timeout-minutes: 45
     defaults:
       run:

.gitea/workflows/windows.yml

@@ -68,8 +68,20 @@ on:
   workflow_dispatch:
 
 jobs:
+  # SECURITY: this job builds PULL-REQUEST code (attacker-controllable build.rs / cargo build) on the
+  # host-mode, persistent `windows-amd64` runner that the release-SIGNING jobs (windows-host.yml /
+  # windows-msix.yml, which decrypt MSIX_CERT_PFX_B64 + REGISTRY_TOKEN to disk) also run on. Untrusted
+  # PR code could therefore persist on that machine or harvest signing material a later job exposes.
+  # The DEFINITIVE fix is operational and lives outside this file: enable Gitea's "require approval to
+  # run workflows for PRs from outside collaborators/forks", and/or route PR CI to isolated ephemeral
+  # runners. The `if:` below is only a backstop — it skips fork PRs where Gitea reports the fork flag,
+  # and FAILS OPEN (still runs) for same-repo PRs and on Gitea versions that don't populate it, so it
+  # never blocks internal PR CI.
   build:
     runs-on: windows-amd64
+    if: >-
+      github.event_name != 'pull_request' ||
+      github.event.pull_request.head.repo.fork != true
     timeout-minutes: 90
     strategy:
       fail-fast: false