Write the warning census and the config differential of the Current set

[tamnd]

gk publish now writes two more pages from the store, both over the newest cell at every crossing of a Current kernel with an upstream GCC column.

reports/warning-census.md is the census of spec 11.5. Per configuration it has a table per GCC column (cells, warnings, warnings per cell, units, options, units under -Werror, fatal), a bar chart of warnings per cell, a table per warning option with the oldest and newest column that gives it, and the warnings on units built with -Werror. So far every published cell is clean: the only "warning" lines in the make logs are kbuild's own sha1sum echo, which never reaches warnings.jsonl.

reports/config-differential.md is the differential of spec 11.3 on the Current set. For each platform and configuration, every kernel's .config is compared with the one of the next column that has a cell, and the symbols that change are gathered per step, with the kernels they changed on and both values. CC_VERSION_TEXT and GCC_VERSION change at every step and are left out. gk config-diff is still there for a single kernel.

The README links both pages under the matrices, and spec 11.3 and 11.5 name the files.

Two things the first run of the differential shows:

• The binutils 2.47 assembler reports itself as 2.47.20260726, and the kernel's as-version.sh reads the date as the patch level, so every cell with binutils 2.47 gets CONFIG_AS_VERSION=20285426. Every AS_VERSION >= test passes either way, so nothing breaks yet. I'll open a finding for it.
• A cell from before Skip KUnit tests in the configuration, not on the command line #65 still has KUNIT_ALL_TESTS=y, so a step between an old and a new cell shows the KUnit options flipping. Those cells have a different config digest and the stripe queues rerun them, so this goes away by itself.