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, 4 insertions, 19 deletions

diff --git a/src/runtime_config.zig b/src/runtime_config.zig
index bd1ba00b..69d2cdfe 100644
--- a/src/runtime_config.zig
+++ b/src/runtime_config.zig
@@ -4,22 +4,7 @@
 //! data it mutates and the Config report reads. Neither contains callbacks.
 const std = @import("std");
 const panel_animation = @import("panel_animation.zig");
-
-/// HOW LONG A HOST-SUPPLIED ABSOLUTE PATH MAY BE, and the only reason this
-/// record was ever kilobytes: the shell a native host resolved, the font file
-/// a native picker returned, and (in pardes.zig) the one watched theme file.
-/// All three name something on a FILESYSTEM, and all three are retained
-/// inline because the core has no allocator at the point they arrive.
-///
-/// Fixed at 4095 wherever a filesystem exists — deliberately NOT derived from
-/// std.fs PATH_MAX, which web has no answer for, and 4095 rather than 4096 so
-/// the macOS C bridge's NUL fits without a second, subtly different limit at
-/// that boundary. Zero on the P4 firmware, which has no filesystem, no
-/// processes to spawn a shell for and no font picker: `Text(0)` is a
-/// zero-sized field whose `set` refuses every non-empty path, so the three
-/// producers report failure instead of storing 12 KiB nothing can fill.
-pub const host_path_cap: usize =
-    if (@import("pardes_config").platform == .p4) 0 else 4095;
+const limits = @import("limits.zig");
 
 /// Tagline glyphs retain body-cell geometry, so allowing a face larger than
 /// the body would clip into neighbouring cells. Zero would make the role
@@ -64,15 +49,15 @@ pub const State = struct {
     shell: struct {
         requested: Text(255) = .{},
         // One data schema across native, browser and freestanding builds; only
-        // its one absolute-path field follows `host_path_cap`.
-        effective: Text(host_path_cap) = .{},
+        // its one absolute-path field follows `limits.host_path_cap`.
+        effective: Text(limits.host_path_cap) = .{},
         pending: bool = true,
     } = .{},
 
     /// The shell acknowledges a font before `effective` changes. A rejected
     /// request therefore remains queryable without claiming it is on screen.
     font: struct {
-        requested_path: Text(host_path_cap) = .{},
+        requested_path: Text(limits.host_path_cap) = .{},
         requested_name: Text(255) = .{},
         effective_name: Text(255) = .{},
         pending: bool = false,