build: pardes builds without the ESP32-P4 toolchain checkout beside it

`build.zig.zon` named `../05-zig-p4` as a path dependency, and `build.zig`
`@import`ed it inside `if (-Desp32p4-firmware)`. But `@import` in a build script
is resolved when the SCRIPT is compiled, not when the branch that needs it is
taken -- so naming the package at all meant anyone without that sibling checkout
could not build pardes AT ALL. Not the firmware: the terminal shell, the SDL
shell, the tests. `zig build` failed with

    build.zig:1073: error: no module named 'zig_p4' available within module 'root.@build'

from a line inside an `if` that was false.

Neither escape hatch works for a PATH dependency, and both were tried rather
than assumed. `.lazy = true` is about FETCHING; a path dep whose directory is
absent is generated as a package with no `build.zig` rather than one marked
unavailable, so `b.lazyImport` -- which exists for exactly this and is what the
standard library says is to `@import` what `lazyDependency` is to `dependency`
-- reaches a `@compileError` instead of returning null. Making it a fetched
dependency instead is not available either: the toolchain has no remote.

So the duplicate goes. That build tree's firmware block linked an image the
toolchain repository already knows how to link -- its own build.zig has
`-Dpardes`, `-Dapp=<root>` and `-Dpardes-obj=<path>`, and its comments record
having learned this same lesson from the other direction, where nesting pardes's
~30-package graph under it broke every build there. The object is the seam: it
crosses by PATH and never by package, and each repository builds what it owns
the pieces of.

  zig build -Dplatform=esp32p4                    # here, no toolchain needed
  zig build -Dpardes                              # there, the console image
  zig build -Dpardes -Dapp=<pardes>/src/esp32p4_9p.zig   # there, the 9P image

For the second and third to work with no module map, `src/board9p.zig` and the
9P firmware root now reach the codec by PATH instead of through a named `ninep`
module that only pardes's own build.zig knew to inject -- which is also why the
root moved from `src/esp32p4/nine.zig` up to `src/esp32p4_9p.zig`, beside
`src/esp32p4.zig`: a path import may not escape its module's own directory. Both
files are now self-contained, and `zig test src/board9p.zig` works with no flags.

`-Desp32p4-port`, `-Desp32p4-prof` and `-Desp32p4-cpu-mhz` go with the block. An
option this build cannot honour is worse than no option, because it accepts the
flag and then ignores it; all three are spelled the same way in the toolchain.

Verified by moving ../05-zig-p4 out of the way: `zig build`, `zig build
-Dplatform=esp32p4` and `zig build unit-test` all pass without it. With it back,
the toolchain still links both images -- console 812,720 B, 9P 88,096 B.

next-steps.txt gains the six features the 9P chain shipped.

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