fix(decoder): give the container the Canary it is supposed to run

The database mount below was one of three ways the decoder could not reach its
own oracle. The other two are here.

`run-canary` never looked in `Checked/`. It tried `Release/` then `Debug/`, and
both of those exist on this box -- an Aug 28 binary and a Jul 19 one. They boot
the game perfectly well and carry NO `audit_61` branch probe, so a probe run
against either returns zero hits that read as a finding about the game rather
than as a stale binary. Configuration is now the outer loop and location the
inner one, so a `Checked` build anywhere beats a `Release` build anywhere;
`$XENIA_BIN` still overrides everything. Measured here: `Checked` has
`audit_61_branch_probe_pcs`, `Release` and `Debug` do not.

The launcher also now says which instrumentation is missing BEFORE the run,
because the alternative is reading an empty log afterwards and guessing.

`build-canary` built `$PROJECT_DIR/xenia-canary`, which does not exist in this
container -- the source is bind-mounted at `/canary` and the launcher already
exports `XENIA_SRC=/canary`. CONTAINER-NOTES has carried that defect since
2026-08-29 with a symlink workaround and a warning to remember to delete the
symlink afterwards. It now reads `$XENIA_SRC` first, so there is nothing to
remember. Its default configuration moves Release -> Checked to match what
`run-canary` picks; the old default spent a full build on a binary nothing ran.

Two documented blockers are refuted rather than deleted, since the sequence of
wrong readings is what makes the right one checkable: the CONTAINER-NOTES
symlink dance (the warm build volume it was configured against is gone too,
removed in the 2026-09-18 cleanup, so the next build configures cleanly against
`/canary`), and `upstream-baseline.md`'s "`version.h` is never generated" --
`CMakeLists.txt` generates it at configure time now, with a stub fallback.

`decoder-loop.md` claimed the oracle was at `Linux/Release/` and that the probe
was on two side branches; both were true when written and neither is now.

Verified: five pick_bin cases against the extracted function body, and `strings`
on all three real binaries.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docker/decoder/bin/build-canary

@@ -10,11 +10,22 @@
 # AVAILABLE MEMORY as well as core count — a full-parallel build of this tree
 # has OOM-killed the host outright.
 #
-#   build-canary [Release|Debug]  [extra cmake --build args]
+# `Checked` is the default, and should stay it: it is what the host builds and
+# what `run-canary` picks first, so the two agree by construction. Building
+# `Release` here instead leaves a second binary that `run-canary` will not use
+# -- hours of CPU for something nothing runs. `Checked` is optimised, with
+# assertions left in.
+#
+#   build-canary [Checked|Release|Debug]  [extra cmake --build args]
 set -euo pipefail
 
-CONFIG="${1:-Release}"; shift || true
-SRC="${PROJECT_DIR:-/work}/xenia-canary"
+CONFIG="${1:-Checked}"; shift || true
+# $XENIA_SRC FIRST. In this container the Canary source is bind-mounted at
+# `/canary`, and `$PROJECT_DIR/xenia-canary` does not exist -- so the old default
+# made this script unusable here, and the documented workaround was to symlink
+# `/canary` into the repository and remember to delete it again (CONTAINER-NOTES
+# §"The toolchain is real"). The launcher already exports the right path; read it.
+SRC="${XENIA_SRC:-${PROJECT_DIR:-/work}/xenia-canary}"
 BUILD="${XENIA_BUILD_DIR:-/sylph-home/re/canary-build}"
 JOBS="${SYLPH_JOBS:-2}"