A function-local static std::mutex has an exit-time destructor, which
the macOS build rejects with -Wexit-time-destructors. The lock is now
allocated once and never destroyed, which also keeps it usable by a
thread that starts a child while the process exits.

Reading the coverage profiles on several threads broke an assumption of
ChildProcess::Start on POSIX: it changes the working directory of the
whole process around posix_spawn. Two gcov runs started at once could
both run in one test's directory, so on the GCC job the map of
"Subtracts one" showed what "Adds one" reached.

- ChildProcess::Start holds a lock while a child starts, on POSIX and
  Windows, so the directory change and the handles a child inherits never
  overlap with another start.
- gcov gets the absolute path of the data file and no working directory,
  so no directory changes while profiles are read in parallel.

- gcov is run with --branch-probabilities; without it the JSON has no
  branches, so GCC runs had none in coverage.json and coverage.lcov.
- PrepareCoverageDirectory also removes coverage.json, coverage.lcov and
  coverage.profdata of an earlier run, so a run that writes no reports
  leaves none to be read as its own.
- WriteCoverageReports returns notes and failures apart. A profile the
  tools cannot read or a report that cannot be written counts as a missing
  report and fails a run whose tests passed; no profiles or missing tools
  stay notes. The paths are announced only for the reports written.
- ProfiledAttempt takes the last attempt whose profile exists, so a final
  retry or repeat that crashed before writing one no longer hides the
  coverage of the attempt before it.
- The per-test llvm-profdata/llvm-cov and gcov runs use as many threads
  as the run had workers; results keep the order of the tests.

Tests: earlier reports removed and no failure without profiles; an
attempt without a profile gives way to an earlier one; the fixture gains a
branch taken one way, checked in the combined LCOV for both LLVM and gcov.
Amalgamation regenerated.

- The 32-bit MSVC jobs failed on C4244: ParseLcov used the uint64_t it
  read as a std::size_t map key. The line number is now converted once,
  explicitly, before it is used.
- extras/catch_amalgamated.cpp is regenerated with
  bazelisk run //tools:amalgamate, so it holds CoverageReport and the
  scheduler changes; bazel run //tools:check passes locally.

--coverage=DIR runs every attempt in a process of its own and keeps the
coverage of each test, for binaries built with LLVM source-based
coverage (Clang: -fprofile-instr-generate -fcoverage-mapping) or gcov
(GCC: --coverage).

How it works:

- The scheduler gives each attempt a fresh worker, as --isolate does,
  and sets LLVM_PROFILE_FILE and GCOV_PREFIX for it: the profile the
  process writes at exit is then the coverage of that one test, in
  DIR/profiles/t<index>-a<attempt>.profraw or a .gcov directory. Each
  runtime reads its own variable, so no symbol of either runtime is
  looked up: weak references would not pull them out of their static
  archives, and Mach-O does not allow undefined weak symbols.
- A process that runs other tests first (bisection) writes beside the
  profiles, never into one. The attempt records its profile, also in
  the JSON results.
- The profiles of an earlier run are removed when the run starts,
  because gcov adds to the .gcda files it finds.
- After the run, CoverageReport reads the last profile of every test:
  llvm-profdata and llvm-cov (PATH, xcrun on macOS, or
  CATCH3_LLVM_PROFDATA / CATCH3_LLVM_COV) export LCOV, gcov (or
  CATCH3_GCOV) writes JSON next to a copy of the .gcno. The result is
  DIR/coverage.json, the files, lines and branches each test reached,
  and DIR/coverage.lcov for the whole run. Without the tools the
  profiles stay and the run says so.
- --coverage with --in-process or --break is refused.

Tests: option parsing, LCOV reading and writing, gcov JSON, and an
integration test on CoverageFixture, a binary CMake builds with the
coverage flags of Clang or GCC: two tests reach different lines, each
map holds only its own, the LCOV report holds both, and a stale profile
of an earlier run is gone. Locally all 84 CTest tests pass with Clang,
and the coverage tests pass with GCC (built with
CATCH3_ENABLE_WERROR=OFF: Catch3 does not build with GCC's warnings as
errors, before and after this change).

CI: the Linux jobs install LLVM's coverage tools, a new job checks the
gcov path with GCC, and CATCH3_COVERAGE_REQUIRE_MAP turns a missing map
into a failure on Linux and macOS instead of a skip. MSVC has no
source-based coverage, so its builds skip those tests; the roadmap
lists step 4 as in progress until Windows with clang-cl is tested.

EnvironmentBlock replaced every non-ASCII character of an inherited name
with '?' before asking whether to withhold it, so --pass-env=MY_?_TOKEN
kept an unrelated MY_é_TOKEN. Names are now converted with Narrow, as
everything else the runner compares.

A block whose variables were all withheld, with no additions, ended in one
terminator; CreateProcessW needs two for an empty block.

The amalgamated sources are regenerated for the runner changes.

Workers and the other processes the runner starts copied its whole
environment, tokens and passwords included. ChildProcess::Start now
leaves out every variable whose name contains, in any case, TOKEN,
SECRET, PASSWORD, PASSWD, CREDENTIAL, API_KEY, APIKEY, PRIVATE_KEY or
ACCESS_KEY (IsSecretVariable), unless ChildOptions::keptEnvironment
names it; names compare without case on Windows.

--pass-env=NAME, repeatable, and "pass-env" in catch3.json keep a
variable for the tests. The runner itself still starts with the whole
environment, so code that runs before the first worker sees it;
docs/progmasoft-runner.md says so.

A runner test starts the shell's environment listing with a secret and
a plain variable set, sees only the plain one, and sees the secret once
it is kept.

A report no longer throws, so a noexcept mock survives an aborting run.
A call that has nothing to return still leaves the test case, which ends
the process in a noexcept function; the documentation says so.

RunContext clears the result capture when it goes, so a mock of static
lifetime finds none and writes to standard error.

Decision owns its source line, since Verify can free the expectation
while a call is on its way. Moving a result out takes the lock. Vector
matchers refer to their vector, so they are rejected at compile time
like composed ones. A null text expectation accepts only null, and
integers compare by value, so 256 no longer accepts an unsigned char.

A static mutex needs an exit-time destructor, which Clang's warnings
refuse, and a mock of static lifetime can outlive it. The test seam is
held the same way, and a test covers a mock method with nothing after
its parameters, which needs an empty variadic argument to stay valid
under -Wpedantic.