feat(v4.0.0): CONSOLE-RX and CONSOLE-STATUS on the single-node model

The console's receive side, standing in for the console node until the
mesh exists, as CONSOLE-TX does for output.  A data fetch (@, @+, @b)
from CONSOLE-STATUS gives -1 when a character is pending and 0 when
not; from CONSOLE-RX it gives the next character and takes it, or -1
with none pending.  The characters come from a queue the test feeds.

Instruction words and literals are still fetched with v4_node_load, so
code at a register's address is never taken for the register.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

v4/README.md

@@ -14,8 +14,9 @@ and every ISA, hosted and bare metal, must still reach its `ok` prompt.
 `make -C v4 test` passing is a development check, not acceptance.
 
 What exists so far is the single node: registers, circular stacks, memory, all
-32 opcodes, per-opcode and per-call-target heat, and a console capture
-register (`CONSOLE-TX`) standing in for the console node until the mesh exists. `make -C v4 test` builds
+32 opcodes, per-opcode and per-call-target heat, and three console
+registers (`CONSOLE-TX`, which captures output, and `CONSOLE-RX` and `CONSOLE-STATUS`, which hand out
+input a test feeds) standing in for the console node until the mesh exists. `make -C v4 test` builds
 and runs the tests at both cell widths; `make -C v4 sanitize` repeats them
 under ASan and UBSan. There is no compiler capsule, no POST and no K
 measurement yet.