Gate mutation testing on a measured score, not an inherited one

The threshold had been 80 against src/stdlib.c alone. There are three
more sources now and nobody had measured them, so that number was a
guess carried forward.

Measured: 72.3%, 188 of a 260-mutant sample from the 1701 the four
sources generate. Gate set to 65 -- a ratchet with headroom for the
runner and for the sample shifting as sources change, not a target.

A sample rather than the whole set, because 1701 rebuilds and test runs
is hours. --max-mutants samples by even index rather than at random, so
the same 260 run every time and the gate stays reproducible; sampling
all four files beats exhausting one of them, which is what this job did
before.

72.3% against the 89.6% reported at 0.1.0 is a change in denominator,
not a regression in the tests. That figure covered one 561-line file;
this covers four totalling 1716 lines, and most of the added surface is
argument validation whose mutants are frequently *equivalent* -- 12 of
the 72 survivors are `errno = 0` deleted from a wrapper whose libc call
always sets errno, which no test that could be written would catch. The
README breaks all 72 down and says which are worth acting on; TODO.md
2.4 carries the three clusters that are.

Two of them were real and are fixed here and in the previous commit: the
right child's `depth + 1` in the depth-first walk, and aksl_tree_remove
on an empty tree, which without its guard dereferences NULL. Neither had
a test; both do now. That is what the harness is for.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.githooks/pre-push

@@ -30,7 +30,7 @@ set -u
 
 # Keep in sync with the --threshold in .gitea/workflows/ci.yaml, so a push that
 # would fail CI fails here first.
-MUTATION_THRESHOLD="${AKSL_MUTATION_THRESHOLD:-80}"
+MUTATION_THRESHOLD="${AKSL_MUTATION_THRESHOLD:-65}"
 
 ZERO_SHA=0000000000000000000000000000000000000000
 
@@ -102,11 +102,15 @@ if [ "${AKSL_HOOK_MUTATION:-0}" = "1" ]; then
     echo "pre-push: mutation testing, threshold ${MUTATION_THRESHOLD}% (this takes a while)"
     # Deliberately not wrapped in run(): this one is slow enough that you want
     # to watch it make progress.
+    # Sampled, like CI: the full set is 1701 mutants and each is a rebuild plus
+    # a full test run. --max-mutants samples by even index rather than at
+    # random, so this is the same 260 CI gates on.
     if ! python3 scripts/mutation_test.py \
             --target src/stdlib.c \
             --target src/string.c \
             --target src/stream.c \
             --target src/collections.c \
+            --max-mutants "${AKSL_HOOK_MUTANTS:-260}" \
             --threshold "$MUTATION_THRESHOLD"; then
         echo >&2
         echo "pre-push: mutation score below ${MUTATION_THRESHOLD}%. Push aborted." >&2