Merge Fn and Sys into a single layer (system actions on hold)

[andersonba]

Summary

Merges the Fn and Sys layers into one. The two were sparse and complementary (Fn = media + F-keys, Sys = bluetooth/bootloader/reset/alpha/lock), so they fit on a single layer without conflicts. Access stays on the Fn combo (LH1+RH1); the old Sys combo (LH0+RH0) is freed.

To avoid mistypes, every former Sys action only fires on hold (tap is inert). Media and F-keys remain plain taps.

Layer layout

 BT0    BT1   BT2   BT3   BTclr      Alpha  F7   F8   F9   F12
 ·      Prev  Play  Next  Boot       Boot   F4   F5   F6   F11
 Lock   Vol-  ·     Vol+  Reset      Reset  F1   F2   F3   F10

• Left = system + media; right = F-keys + Alpha.
• BT0–3, BTclr, Boot, Reset, Alpha, Lock → hold-only (200ms, hold-preferred).
• Boot/Reset are duplicated (inner columns of both halves) so either half can enter bootloader/reset.

Implementation notes

• New HOLD_ONLY(name, action) helper wraps each system action as a hold-tap with tap = &none.
• &bt takes two params and can't go directly into a hold-tap (ZMK limitation), so it's wrapped: bt_clr (macro) and bt_sel_n (one-param macro for the profile index).
• Removing the Sys layer reindexes MOUSE (6→5) and SCROLL (7→6). Everything in code uses the layer names, so only the #defines changed — except the one hardcoded index in crosses_right.overlay (layers = <7> → <6>), updated here.
• Orphan _LH2_Sys / _RH2_Sys extra-key defines removed.

Affects all boards

config/base.dtsi is shared across Urchin, Corne and Crosses. CI builds all of them.

Not included

• draw/*.svg were not regenerated (no local keymap-drawer env), so the visualizations still show the old Fn/Sys split.

Draft until the CI build passes (couldn't compile locally — ZMK workspace not set up).