zig-p4: pure-Zig ESP32-P4 toolchain

build.zig generates the linker script and drives Zig's own LLD; tools/image.zig
turns the ELF into a flashable image and tools/{rom,serial}.zig speak the mask
ROM loader over the UART. No CMake, ninja, idf.py, esptool, or external linker.

src/soc.zig is a comptime register model over ESP-IDF's own *_reg.h headers;
src/hal/ adds peripheral sequences; src/io/ implements std.Io for the chip;
src/oracle/ diffs this HAL against ESP-IDF's on the die.

1 files changed, 19 insertions, 0 deletions

diff --git a/src/oracle/runtime_ref.c b/src/oracle/runtime_ref.c
new file mode 100644
index 0000000..659661b
--- /dev/null
+++ b/src/oracle/runtime_ref.c
@@ -0,0 +1,19 @@
+/* The few libc symbols ESP-IDF's LL code reaches for, supplied so the reference can link into a
+ * freestanding image.
+ *
+ * There is exactly one so far, and it is reached by design rather than by accident:
+ * `_uart_ll_set_baudrate` (uart_ll.h:532-535) calls `abort()` when handed an LP_UART instance,
+ * because that path needs `lp_uart_ll_set_baudrate` instead. The differential harness only ever
+ * passes HP UART instances, so this is unreachable in practice - but the linker does not know that,
+ * and a missing `abort` fails the build with a symbol name that explains nothing about why.
+ *
+ * Spinning rather than resetting is deliberate: if a reference implementation ever does call this,
+ * the board stops with its last console line intact, which is the difference between a diagnosable
+ * failure and a reboot loop.
+ */
+
+__attribute__((noreturn)) void abort(void)
+{
+    for (;;) {
+    }
+}