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, 9 insertions, 41 deletions

diff --git a/src/allocators.zig b/src/allocators.zig
index cf024a0b..61c55f39 100644
--- a/src/allocators.zig
+++ b/src/allocators.zig
@@ -1,43 +1,9 @@
 const std = @import("std");
 const builtin = @import("builtin");
-const config = @import("pardes_config");
+const limits = @import("limits.zig");
 
 const Allocator = std.mem.Allocator;
 const debug_enabled = builtin.mode == .Debug;
-/// Three tiers, because the address space differs by four orders of magnitude.
-/// `reduced_target` is the browser: a wasm linear memory it grows on demand, so
-/// the static reservations are megabytes rather than tens.
-///
-/// `p4` is ESP32-P4 firmware, and its tier is deliberately ALL FALLBACK. Every
-/// capacity here is a `StackFallbackAllocator`'s buffer, which is a static and
-/// therefore lands in `.bss` — and on the P4 `.bss`, `.data` and the stack all
-/// share ONE 240 KiB chunk of L2MEM at 0x4FF03000, while the heap the fallback
-/// allocator hands out is the separate 384 KiB chunk at 0x4FF40000 - the 128 KiB
-/// above that measured as L2 cache rather than memory. A
-/// megabyte-shaped reservation here would not fit, and every byte that did fit
-/// would be taken from the stack's neighbourhood to duplicate memory the heap
-/// already has. So the buffers exist only because the type requires one: 4 KiB
-/// absorbs the small churn, and everything else spills to the real heap on the
-/// first allocation.
-const p4 = config.platform == .p4;
-const reduced_target = config.platform == .web or builtin.os.tag == .freestanding;
-const KiB = 1024;
-const MiB = 1024 * KiB;
-
-const capacities = struct {
-    const pardes = if (p4) 4 * KiB else if (reduced_target) 8 * MiB else 32 * MiB;
-    const frame = if (p4) 4 * KiB else if (reduced_target) 4 * MiB else 16 * MiB;
-    // Zero is legal and always spills, which is exactly what an arena for a
-    // compiled-out subsystem should do. `StackFallbackAllocator(0).buffer` is
-    // `[0]u8`; `get()` inits the FixedBufferAllocator over an empty slice, so
-    // `FixedBufferAllocator.alloc` fails every nonzero request and `alloc`
-    // falls through to `self.fallback_allocator.rawAlloc`, while `ownsPtr` over
-    // an empty range is false for every pointer so `resize`/`remap`/`free`
-    // route to the fallback too. See lib/std/heap.zig, StackFallbackAllocator.
-    const tree_sitter = if (p4) 0 else if (reduced_target) 4 * MiB else 16 * MiB;
-    const image = if (p4) 0 else if (reduced_target) 64 * KiB else 32 * MiB;
-    const pdf = if (p4) 0 else if (reduced_target or !config.mupdf) 64 * KiB else 64 * MiB;
-};
 
 pub const Allocators = struct {
     pardes: Allocator,
@@ -48,11 +14,13 @@ pub const Allocators = struct {
     pdf: Allocator,
 };
 
-var pardes_fallback: std.heap.StackFallbackAllocator(capacities.pardes) = undefined;
-var frame_fallback: std.heap.StackFallbackAllocator(capacities.frame) = undefined;
-var tree_sitter_fallback: std.heap.StackFallbackAllocator(capacities.tree_sitter) = undefined;
-var image_fallback: std.heap.StackFallbackAllocator(capacities.image) = undefined;
-var pdf_fallback: std.heap.StackFallbackAllocator(capacities.pdf) = undefined;
+/// The static reservations, one per profile tier. See `limits.arena` for why
+/// each number is what it is, and why the board's are 4 KiB and zero.
+var pardes_fallback: std.heap.StackFallbackAllocator(limits.arena.pardes) = undefined;
+var frame_fallback: std.heap.StackFallbackAllocator(limits.arena.frame) = undefined;
+var tree_sitter_fallback: std.heap.StackFallbackAllocator(limits.arena.tree_sitter) = undefined;
+var image_fallback: std.heap.StackFallbackAllocator(limits.arena.image) = undefined;
+var pdf_fallback: std.heap.StackFallbackAllocator(limits.arena.pdf) = undefined;
 
 const Debug = std.heap.DebugAllocator(.{});
 var pardes_debug: Debug = .init;
@@ -121,7 +89,7 @@ test "fixed allocators are separate, spill, and restart" {
     try std.testing.expect(frame_fallback.fixed_buffer_allocator.ownsPtr(frame.ptr));
     try std.testing.expect(core.ptr != frame.ptr);
 
-    const spill = try allocs.pardes.alloc(u8, capacities.pardes + 1);
+    const spill = try allocs.pardes.alloc(u8, limits.arena.pardes + 1);
     try std.testing.expect(!pardes_fallback.fixed_buffer_allocator.ownsPtr(spill.ptr));
     allocs.pardes.free(spill);
     allocs.frame.free(frame);