bwn BCM43224 (MacBook Air A1370) driver fixes for FreeBSD 15.1

Patches + build scripts + firmware sources to make the Broadcom BCM43224
(PCI 0x14e4:0x4353, subvendor 0x106b:0x00d1) work on FreeBSD 15.1.

Fixes:
- Fix 1: compile GPL N-PHY code (BWN_GPL_PHY build wiring)
- Fix 2: NULL-guard bwn_dma_ringfree on partial attach
- Fix 3: wire bwn_update_mcast to ic->ic_update_mcast
- Fix 5: load bwn_v4_n firmware at loader stage (loader.conf)
- Fix 6: reclaim pending TX frames at bwn_dma_stop (detach double-free)
- Fix 7: driver-owned node ref + remove dr_usedslot early-return +
        bhndb_pci BHNDB_PCI_QUIRK_NO_MSI for BCM43224 (legacy INTx)
- Fix 8: restrict reclaim to TX rings only (RX reclaim caused heap
        corruption / TCP panic)

README.md  = rebuild guide (apply.sh + build.sh, manual steps)
RESEARCH.md = root-cause investigation log (Fixes 5-8)

src/firmware/Makefile

@@ -0,0 +1,9 @@
+KMOD=	bwn_v4_n
+FIRMWS=	ucode16_mimo.fw:bwn_v4_ucode16_mimo \
+	n0initvals11.fw:bwn_v4_n0initvals11 \
+	n0bsinitvals11.fw:bwn_v4_n0bsinitvals11 \
+	n0initvals16.fw:bwn_v4_n0initvals16 \
+	n0bsinitvals16.fw:bwn_v4_n0bsinitvals16 \
+	n0initvals24.fw:bwn_v4_n0initvals24 \
+	n0bsinitvals24.fw:bwn_v4_n0bsinitvals24
+.include <bsd.kmod.mk>