bba_register_file: delete the dead rx_rptr FIFO and unused constants

An audit for signals that are written but never read, ports nothing wires, and
constants only the tests touch turned up something bigger than stray signals:
the whole rx_rptr SyncFIFO was dead.

asm.rrp is wired from reg.rrp -- the combinational tap -- and the FIFO's far
side (rx_rptr_r_data / _r_en / _r_rdy) is wired NOWHERE in BBATop.  It is a
leftover from when the register file and the frame assembler sat in different
clock domains; when those merged the direct tap replaced it and the FIFO was
never removed.  Gone: a 4x8 SyncFIFO, its write logic in the RRP case, its
default strobe and three ports.

Also removed _RWP_HI and _RRP_HI, which were never referenced.  The register
table now says why instead: the ring is 15 pages, so the high bytes of both
pointers are always zero and the read mux's default already returns that.

NCRB_PR went the other way -- it was used only by a test while BBATop steered
the ethernet address filter from a bare `reg.ncrb[0]`.  It is now public and
BBATop imports it, so the constant is live and the magic index is gone.

Kept deliberately: _NCRB_PM and _NCRB_AB are referenced only by T15, but they
document the filtering bits of a real register and make that assertion
readable, so they stay.

Area went DOWN, which is the evidence the FIFO was real logic rather than
something yosys had already pruned:

    ICESTORM_LC   4350 original -> 4380 after the docs pass -> 4328 now

Full suite passes, bba_top passes at EXI_HALF=1/2/8.

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

exi_bba/bba_top.py

@@ -16,7 +16,7 @@ See CLAUDE.md "Module Breakdown" and "CDC Signal Inventory" for the full list.
 from amaranth import *
 
 from exi_bba.exi_capture           import ExiCapture
-from exi_bba.bba_register_file     import BBARegisterFile
+from exi_bba.bba_register_file     import BBARegisterFile, NCRB_PR
 from exi_bba.spram_arbiter         import SPRAMArbiter
 from exi_bba.rx_frame_assembler    import RXFrameAssembler
 from exi_bba.tx_frame_drain        import TXFrameDrain
@@ -318,7 +318,7 @@ class BBATop(Elaboratable):
         # ask for promiscuous by setting PR.  See the S0_MODE comment in
         # w5100_parallel_master.py -- running unfiltered hung Swiss.
         if self._eth == "w5100":
-            m.d.comb += eth.promisc.eq(reg.ncrb[0])
+            m.d.comb += eth.promisc.eq(reg.ncrb[NCRB_PR])
 
         # ── Optional bring-up peripherals (sync domain) ───────────────────
         # Build init_req as an OR of all reinit sources (NCRA pulse plus any