Describe the two legs the gate section left out [#326]

CONTRIBUTING.md

@@ -249,6 +249,39 @@ prove them. It is the one leg whose compiler is not the pinned one, for the reas
 `rust-toolchain.toml` gives, and it names its own toolchain and target and prints
 both on every run.
 
+**`coverage`** measures line coverage with the compiler's own instrumentation and
+judges it on the surface `.github/coverage/pinned-surface` names. That file is a
+register of the kind `.github/invariants/rules` is - a kind, a path and the
+reason on one line - and a line missing any of the three is refused, as is an
+`area` naming nothing tracked and a tracked source file under an `area` that no
+`module` line lists. The bar is on the modules the register names rather than on
+the whole tree, and the number is in the script with the measurement it was set
+from rather than here: a whole-repository percentage is met by covering the easy
+half and missed by adding a large trivial module, so a bar over it cannot do what
+#84 asks of one. The whole-tree figure is measured and printed on every run and
+gates nothing, which is the half that is tracked rather than enforced. It reads
+the counters with the tools in the pinned toolchain's sysroot rather than
+whatever LLVM answers on the path, because a system LLVM of another version
+answers differently and a verdict that depends on who ran it is not a verdict.
+`.github/coverage/coverage.sh` holds the rules and the fixtures that prove them.
+
+**`mutation`** changes the code under the suite and reports which changes the
+suite failed to notice, which is the question neither `test` nor `coverage` can
+ask: a test that reaches a line and asserts nothing about the answer is green in
+both. It runs weekly and on request rather than in front of a merge, because
+every mutant is a build and a test run of its own, and IT GATES NOTHING. A
+survivor obliges an issue on this board carrying the mutant in the analyser's own
+words, the module it is in, and what would have caught it - not a re-run and not
+a quiet acceptance, because a number produced on a schedule that nobody must
+answer is a number nobody reads. There is deliberately no suppression register
+beside it: excusing a mutant raises the published score with no test written,
+which moves the number in the one direction that looks like progress and is not,
+so a mutant nobody can kill is written into its issue with that reason and the
+issue closed on it. Its scope is the `area` lines of the same register `coverage`
+pins its bar to, so a source file added under one is mutated whether or not
+anybody remembered it. `.github/mutation/mutation.sh` holds the rules and the
+fixtures that prove them.
+
 **`dependencies`** restores in locked mode, so a restore that would rewrite
 `Cargo.lock` fails rather than proceeding quietly, and scans the graph the
 committed lockfile declares against the advisory database. It reads back how many
@@ -297,8 +330,13 @@ triples come from the `targets` leg's own reader through
 `.github/cross-toolchain/cross-toolchain.sh` holds the rules and the fixtures
 that prove them.
 
-THREE RUNS REPORT AND REFUSE NOTHING, which is deliberate rather than an
-oversight, AND THIS PARAGRAPH SAID TWO. **`External addresses in documents`**
+FOUR RUNS REPORT AND REFUSE NOTHING, which is deliberate rather than an
+oversight, AND THIS PARAGRAPH SAID TWO AND THEN THREE. Each move was a leg
+arriving rather than a decision changing, and this one is `mutation` above, which
+stood in the tree for twenty days before this section named it at all. The number
+is a hand count over the entries in this section and no command derives it, so it
+is right about what is described here and says nothing about what the tree
+carries. **`External addresses in documents`**
 requests the addresses documents name and prints what answered; an address
 outside this repository that is down for an hour is not a defect here, and a gate
 that reddens for it teaches people that red means nothing. **`Scorecard