One core behind N frontends, the board's own runner moved in, and every board cap on one screen

## The wire is the effect stream, not a new protocol

`pardes --detach` leaves a core running with no terminal; `pardes --attach` is a frontend that owns
a terminal and a socket and nothing else. N frontends on one core all look at the same screen —
`screen -x`, not N sessions.

The codec (`src/detached/wire.zig`) carries exactly one `Event` or one `Host.VTable` call per
message. That is not a coincidence and it is why there is no third vocabulary to keep in step: the
core's IO seam was already a struct of function pointers with plain-data arguments, so a socket is
a legal implementation of it. `nested.zig`'s socket could not be reused — it carries a builtin
command line, and a command line cannot carry a frame.

ARCHITECTURE-NEUTRAL on purpose, not as decoration. The frontend on the far end may be
riscv32-freestanding on the ESP32-P4 while the core is x86_64 Linux, so every field is an explicit
little-endian fixed width and no message is a blit of a native struct. A protocol that only works
between two builds of the same compiler would have thrown away the one frontend that motivated it.

## The board comes in; its toolchain stays out

`src/p4.zig` becomes `src/esp32p4.zig`, and the pardes half of `../05-zig-p4` — the vaxis-over-
serial runner, the UART editor terminal, the keystroke rescue ring, the on-die test suite — moves
into `src/esp32p4/`. `build.zig.zon` gains `.zig_p4 = .{ .path = "../05-zig-p4" }`, so
`zig build -Dplatform=esp32p4 -Desp32p4-firmware` builds, flashes, monitors and self-tests the
board from this repo's `build.zig`.

The DIVISION is the point. What moved is what only pardes wants: the runner that drives a pardes
core over a serial line. What stayed is everything a second project would also want — the HAL, the
register/radio/oracle layers, the linker script, `_start`. `zig_p4` declares no dependencies of its
own and its `build()` early-returns when it is not the root package, so this costs the package
graph exactly zero packages and the editor's own builds nothing at all.

## limits.zig: nine forgettable places become one budget

Nine `platform == .esp32p4` capacity tests lived in nine files. They were never nine decisions —
they are ONE decision, how much memory this build may spend, taken nine times where no reader could
see the total. `src/limits.zig` puts the whole budget on one screen with every cap named against
what it is measured against, derived from two booleans.

The payoff is testability on a machine that is not the board: the caps are ordinary comptime values,
so a host build can be compiled against the board's numbers and the parking, eviction and clamping
paths a 240 KiB core takes get exercised by the normal test suite instead of only over a UART.

## A bare `zig build`

`zig build` with no arguments now builds the tty and GUI binaries and installs them into
`~/.local/bin`, and says so once on stdout with the flag that overrides it. The old default built
one binary into `zig-out` — a path nothing on a `PATH` ever looks at, which made "build it" and
"use it" two different commands for no reason.

1 files changed, 7 insertions, 17 deletions

diff --git a/src/dump.zig b/src/dump.zig
index 37ba83c0..f8740cdb 100644
--- a/src/dump.zig
+++ b/src/dump.zig
@@ -1,5 +1,6 @@
 const std = @import("std");
 const builtin = @import("builtin");
+const limits = @import("limits.zig");
 
 // scoped, not bare std.log: main.zig's logFn drops the unscoped .default scope
 // wholesale (ghostty and uucode log there too), and a corrupt dump's parse
@@ -40,22 +41,11 @@ pub const magic = "pardes-dump";
 pub const version: u32 = 1;
 pub const max_panes: usize = 16;
 pub const max_cols: usize = 6;
-/// Bounds the only user-editable, schema-owned tag fragment. Keep this beside
-/// the dump limits so readers can reject data before copying it into a pane.
-/// It IS the storage bound: `Pane.tag_tail` is `[max_tag_tail]u8`, and every
-/// writer (appendTag, tagInsert, restoreDumpTail, the acmefs `tag` file)
-/// refuses input that does not fit rather than truncating it, so the schema
-/// limit and the buffer can never disagree.
-///
-/// 512 on the P4 firmware. A tag is ONE line — a pane's path plus its command
-/// words — and 4 KiB of it is 4 KiB per pane out of a 384 KiB heap. A serial
-/// console is 80 columns; 512 is six of those.
-pub const max_tag_tail: usize =
-    if (@import("pardes_config").platform == .p4) 512 else 4096;
 /// Output arguments are typed in the same bounded one-line tag storage. Keep
 /// the schema limit named independently so a dump reader can validate it
-/// without importing the output-pane implementation.
-pub const max_origin_arg: usize = max_tag_tail;
+/// without importing the output-pane implementation. The bound itself is
+/// `limits.max_tag_tail` — the one place a board-shaped capacity is chosen.
+pub const max_origin_arg: usize = limits.max_tag_tail;
 
 pub const Size = struct {
     cols: u16,
@@ -157,7 +147,7 @@ pub fn validate(state: State) !void {
         if (!std.math.isFinite(pane.vweight) or pane.vweight <= 0)
             return error.BadDumpPaneWeight;
         if (pane.scroll > std.math.maxInt(i32)) return error.BadDumpPaneScroll;
-        if (pane.tag_tail) |tail| if (tail.len > max_tag_tail)
+        if (pane.tag_tail) |tail| if (tail.len > limits.max_tag_tail)
             return error.BadDumpTagTail;
         if (pane.file) |file| if (file.origin_arg.len > max_origin_arg)
             return error.BadDumpOriginArg;
@@ -412,9 +402,9 @@ test "validation bounds pane restore state" {
     try std.testing.expectError(error.BadDumpPaneScroll, validate(state));
     panes[0].scroll = 0;
 
-    var oversized_tail: [max_tag_tail + 1]u8 = @splat('x');
+    var oversized_tail: [limits.max_tag_tail + 1]u8 = @splat('x');
     panes[0].tag_tail = &oversized_tail;
     try std.testing.expectError(error.BadDumpTagTail, validate(state));
-    panes[0].tag_tail = oversized_tail[0..max_tag_tail];
+    panes[0].tag_tail = oversized_tail[0..limits.max_tag_tail];
     try validate(state);
 }