ISO keyboards have two physical keys that ANSI does not — POUND (#) at
row 3 col 12 between the right-of-quote position and Enter, and
ISO_BACKSLASH (<) at row 4 col 1 between LShift and Z. The firmware
reports them as zones 47 and 97 on G915 ISO models, but MAIN_ISO only
*subtracted* the ANSI backslash at row 2 col 13 without ever adding
those two cells back. They fell through to the unmapped pool and got
dropped by the EXTRAS_ALLOWLIST phantom-zone filter, so PerKey lighting
silently left them undrawable (issue #3239 — German G915).
Add both cells to MAIN_ISO with the UK QWERTY labels (# and \\) as the
default, and override them in the regional layouts: # / < on QWERTZ DE,
* / < on AZERTY FR. UK QWERTY inherits the defaults.
ANSI is unaffected — MAIN_ANSI still omits 47/97 so they keep getting
filtered as phantoms on ANSI keyboards like the G515.
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