ci(security): add a supply-chain gate, checksums and an SBOM

The project shipped signed Android builds and unsigned desktop binaries
with no vulnerability scanning of any kind. Nothing checked the ~500
crate Rust graph or the JS packages against an advisory feed, and nothing
checked that what we redistribute inside an MIT bundle permits it.

The first cargo-deny run found eight vulnerabilities and one
unsoundness -- bytes, four in rustls-webpki, time, two in quick-xml and
rand -- every one of them closed by a `cargo update` nobody had a reason
to run. That update is in this commit; 740 Rust tests and clippy
-D warnings pass on the new lockfile.

Two structural fixes matter as much as the gate itself:

  - deny.toml scopes the graph to the targets we actually ship. Without
    it the Apple targets pull in plist -> quick-xml and report two DoS
    advisories against a crate that is in no binary we release. Ignoring
    those by ID would silence them everywhere, including where they
    would matter; scoping makes them correctly absent.

  - libmpv is pinned by rev instead of branch = "master". A branch means
    the revision is whatever Cargo.lock happens to hold and any
    `cargo update` silently substitutes new upstream code -- in the one
    dependency that is not from crates.io and that links a C library
    into the player. The rev is the commit already locked, so this pins
    current behaviour rather than changing it.

Licence findings are recorded rather than waved through. libmpv and
libmpv-sys are LGPL-2.1, satisfied here by dynamic linking against the
system library; deny.toml carries the two obligations that follow (keep
the linkage dynamic, ship libmpv's licence text with any bundle carrying
the .so). MPL-2.0 crates are file-level copyleft and fine unmodified.

Releases now publish SHA256SUMS (verified in-job with `sha256sum -c`
before upload) and a CycloneDX SBOM for both halves, so "does this
release contain <vulnerable crate>?" has an answer that is not "rebuild
the tag and re-resolve it".

Workflows pin jellytau-builder:2026.08 instead of :latest. While every
job said :latest, rebuilding the image changed what every build compiled
against, including rebuilds of old release tags.

Also folded in, because both were the same class of problem:

  - publish-docs.yml downloaded mdBook from GitHub releases into
    /usr/local/bin at job time -- a toolchain install in CI, which
    CLAUDE.md explicitly forbids, and a hard dependency on GitHub's CDN
    at publish time. It is in the builder image now.

  - extract-traces.ts only ever read .ts/.svelte/.rs, so every
    requirement implemented by *configuration* was invisible to the
    matrix that measures it. DR-205, DR-206, DR-207 and DR-215 all carry
    TRACES comments nothing read, and each counted as uncovered while
    being covered. Coverage was really 90%, not 88%; MIN_THRESHOLD moves
    to 89 accordingly. CI workflows stay excluded and there is a test
    saying why: traceability-check.yml quotes "a TRACES: comment" beside
    deliberately-undefined example IDs, which the extractor would read
    as real traces and then fail its own dangling-ID check.

Supply-chain requirement is DR-216.

🔴 The builder image must be rebuilt and pushed
(scripts/build-builder-image.sh 2026.08) before this reaches master --
the workflows now name a tag and tools that do not exist in the registry
yet.

.gitea/workflows/traceability-check.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: linux/amd64
     name: Check Requirement Traces
     container:
-      image: gitea.tourolle.paris/dtourolle/jellytau-builder:latest
+      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08
 
     steps:
       - name: Checkout repository
@@ -94,7 +94,7 @@ jobs:
           #
           # Keep in sync with MIN_COVERAGE_PERCENT in scripts/extract-traces.ts;
           # scripts/extract-traces.test.ts fails if the two drift apart.
-          MIN_THRESHOLD=88
+          MIN_THRESHOLD=89
           if [ "$COVERAGE" -lt "$MIN_THRESHOLD" ]; then
             echo "❌ ERROR: Coverage ($COVERAGE%) is below minimum threshold ($MIN_THRESHOLD%)"
             exit 1