Bug-fix sweep: repl reentrancy, virtio/blocksys bounds, identity CRCs, LOG_LINE_MAX

Code review fixes, all compile clean (hosted gcc + aarch64/riscv64 kernel flags):

- repl.c (H1): reentrancy guards on the MSG-TICK idle pump. sk_repl_idle()
  now defers when Hera is mid-interpret (g_mama_interpreting) or when its
  own vm_interpret is on the stack (g_idle_pump_active), so a blocking
  KEY/EXPECT/QUERY inside a dispatched line can no longer re-enter the
  interpreter and clobber the in-flight input buffer.
- virtio_rng.c: clamp device-returned used_len to VRNG_BUF_SIZE before the
  caller's data_buf copy, closing a device-controlled OOB read.
- block_subsystem.c: first-write path now keys off created_time==0 instead
  of dead magic==0 so fresh blocks get a real created_time stamp; first_free/
  last_allocated fixed to absolute Forth LBNs (set in blk_compute_fresh_geometry
  from slot->start_lbn, no longer the wrong physical-BAM-index values from
  compute_totals_from_B); physical-bounds guard on blk_meta_zone_read/write
  prevents unsigned underflow on a corrupt fence >= device size.
- capsule_zuse_boot.c / capsule_wirebind.c: identity seed validated magic ->
  version -> CRC-64 (compute_crc64 over offsetof(crc)) before trusting it,
  so a corrupt/format-mismatched record is refused, never loaded.
- log.h / starkernel/log.h: unused LOG_LINE_MAX 256 renamed LOG_MSG_LINE_MAX
  to lift the include-order collision with vm.h's LOG_LINE_MAX 64; stale
  include-order comments dropped (kernel_main.c, shim.c, capsule_birth.c).
- FABRIC-3.md: three stale-doc carry-forward items closed [x] with cbe7b49
  notes.

Real KEY/?TERMINAL/QUERY/EXPECT bodies (console WIP):
- repl.h/repl.c: sk_console_getkey()/sk_console_key_available()/
  sk_console_readline() public bodies; non-destructive peek buffers the
  found byte so a following KEY returns it.
- shim.c: getchar()/fgetc()/fgets()/sf_terminal_ready() routed through the
  real console paths instead of stubs; sf_terminal_ready() in platform_io.h
  with sf_terminal_ready() implemented for the hosted build (linux/io.c,
  POSIX select on fd 0) wired into Makefile.
- io_words.c: ?TERMINAL now returns actual terminal-readiness, not constant 0.

Artifacts: minted disk/artemis.img + rebuilt lfs kernel; BLOCK_MAP.md,
doe csv + qemu log regenerated.

src/platform/linux/io.c

@@ -0,0 +1,49 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  This file is part of the StarForth project.
+
+  Licensed under the StarForth License, Version 1.0 (the "License");
+  you may not use this file except in compliance with the License.
+
+  You may obtain a copy of the License at:
+      https://github.com/star.4th@proton.me/StarForth/LICENSE.txt
+
+  This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
+  express or implied, including but not limited to the warranties of
+  merchantability, fitness for a particular purpose, and noninfringement.
+
+ See the License for the specific language governing permissions and
+ limitations under the License.
+
+ */
+
+/*
+ * StarForth Platform IO - Linux/POSIX Backend
+ *
+ * Implements platform_io.h's terminal-readiness check using select() on
+ * fd 0, the standard POSIX non-blocking-stdin-peek idiom. Zero timeout:
+ * a single poll, never blocks.
+ *
+ * License: See LICENSE file
+ */
+
+#include "../../../include/platform_io.h"
+#include <sys/select.h>
+#include <unistd.h>
+
+int sf_terminal_ready(void) {
+    fd_set readfds;
+    struct timeval tv;
+
+    FD_ZERO(&readfds);
+    FD_SET(STDIN_FILENO, &readfds);
+    tv.tv_sec = 0;
+    tv.tv_usec = 0;
+
+    int r = select(STDIN_FILENO + 1, &readfds, NULL, NULL, &tv);
+    return (r > 0 && FD_ISSET(STDIN_FILENO, &readfds)) ? 1 : 0;
+}