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, 9 deletions

diff --git a/src/source_manifest.zig b/src/source_manifest.zig
index e2cc312c..c3bfde88 100644
--- a/src/source_manifest.zig
+++ b/src/source_manifest.zig
@@ -15,18 +15,13 @@
 //! Paths are as a user would type them, repo-root-relative, which is what
 //! `look` resolves a click against.
 
-//! ON THE P4 the allowlist is EMPTY, and that is the whole difference: the
-//! table is ~0.95 MiB of rodata against a 1.5 MiB flash partition, and the
-//! firmware's filesystem is the serial host's, reached through the Host
-//! vtable. The API is unchanged — `all` is a zero-length array and `find`
-//! answers null — so every caller compiles identically and simply finds
-//! nothing embedded.
+const limits = @import("limits.zig");
 
 pub const Source = struct { path: []const u8, contents: []const u8 };
 
-/// A slice, not an array: the P4 table is empty and every consumer only ever
-/// iterates or takes `.len`.
-pub const all: []const Source = if (@import("pardes_config").platform == .p4) &.{} else &allowlist;
+/// A slice, not an array: the P4 table is empty (see `limits.embedded_sources`
+/// for why) and every consumer only ever iterates or takes `.len`.
+pub const all: []const Source = if (limits.embedded_sources) &allowlist else &.{};
 
 const allowlist = [_]Source{
     .{ .path = "build.zig", .contents = @embedFile("root-build.zig") },