The API's coverage is measured, and every suite's is in Grafana (#394)

The cargo tests put web.rs at 17% and daemon.rs at 10%, because the
browser suite, which drives the API, ran a daemon nobody measured.
`sh scripts/coverage.sh api` builds an instrumented ipx and
ipx-directory in the capped coverage container and runs the browser
suite here against them (IPX_API_COVERAGE): web.rs is at 80% and
daemon.rs at 77%. What it does not reach: access.rs (Cloudflare Access,
0%; its unit tests 73%) and the directory's public pages (site.rs, 4%).

The coverage image is Ubuntu 24.04 now, as code-server is: built on
Debian trixie, the binaries needed glibc 2.41 and code-server has 2.39.
In that mode the suite starts those binaries rather than building its
own, its tests' `ipx` commands too (global-setup's IPX_BIN), passes
LLVM_PROFILE_FILE to both servers, and stops them with SIGTERM, since
an instrumented binary writes its profile as it exits and Playwright
otherwise kills them outright.

Reports go to target/coverage/cargo, api and page. Each run ends with
scripts/coverage-metrics.ts, which writes every suite's lines covered
and in all, by crate, to node-exporter's textfile directory; the
monitoring project's node-exporter now reads it (one flag, its compose
file backed up beside it), so Prometheus keeps them as
ipx_coverage_lines, and the iPX dashboard's Coverage row draws them:
each suite now, over time, and by part. The cargo tests' totals hold
their own code, which runs whole, so they read higher than the code
alone; the panels say so.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile.coverage

@@ -2,17 +2,22 @@
 # toolchain, LLVM's coverage tools and cargo-llvm-cov. No source: coverage.sh mounts the tree.
 # Built and pushed like the services, by scripts/build-image.sh coverage, under its caps.
 #
+# Ubuntu 24.04, as code-server is (#394): `coverage.sh api` builds an instrumented ipx and
+# ipx-directory here, under the cap, and the browser suite runs them in code-server, which a
+# binary linked against Debian trixie's newer glibc (2.41 to Ubuntu's 2.39) would not start in.
 # The same Rust as the tests are built with elsewhere: a newer one would rebuild everything with
 # its own flags for nothing.
-FROM rust:1.98.1-slim-trixie
+FROM ubuntu:24.04
+ENV RUSTUP_HOME=/usr/local/rustup CARGO_HOME=/usr/local/cargo PATH=/usr/local/cargo/bin:$PATH
 # cc for rusqlite's bundled SQLite and librqbit, as Dockerfile's builder; ffmpeg for DeadAir's
-# tests, as the image ipx runs in.
-RUN apt-get update && apt-get install -y --no-install-recommends \
-        build-essential ffmpeg \
+# tests, as the image ipx runs in; curl and certificates to fetch rustup.
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
+        build-essential ca-certificates curl ffmpeg \
     && rm -rf /var/lib/apt/lists/* \
+    && curl -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal --default-toolchain 1.98.1 --no-modify-path \
     && rustup component add llvm-tools-preview
 # cargo-llvm-cov's own build, as its README gives it, rather than compiling it here.
 ADD https://github.com/taiki-e/cargo-llvm-cov/releases/latest/download/cargo-llvm-cov-x86_64-unknown-linux-gnu.tar.gz /tmp/llvm-cov.tar.gz
 RUN tar -xzf /tmp/llvm-cov.tar.gz -C /usr/local/cargo/bin && rm /tmp/llvm-cov.tar.gz \
-    && cargo llvm-cov --version
+    && cargo llvm-cov --version && rustc --version
 WORKDIR /work