Never build the P4 object Debug; the measured mode becomes the default

`-Doptimize` defaults to Debug, so the naive `zig build -Dplatform=p4` produced an
object that CANNOT RUN. Debug wraps every tier in `allocators.zig` in a
`DebugAllocator`, whose metadata is page-granular, and one 4 KiB page per size class
does not fit in the 384 KiB the board hands the editor: the image links, flashes, and
then dies in `Pardes.init`. Nothing said so, because every build in this session
happened to pass `-Doptimize=` explicitly.

The p4 target now falls back to ReleaseFast, and that mode was measured rather than
preferred. On the die, against ReleaseSmall over 5 document lengths x 7 trials:

    configuration        fixed    per char   at 160 chars
    ReleaseSmall      16.99 ms     54.3 us       25.56 ms
    ReleaseFast       14.85 ms     34.7 us       20.30 ms   0.79x

13% off the fixed per-keystroke cost, 36% off the per-character cost, for 35% more
flash on a partition that is 39% used. An explicit `-Doptimize=` still wins, so
ReleaseSmall stays one flag away when flash matters more than latency - which is why
this is a fallback and not a hard override.

Following the file's own convention: the web target has pinned ReleaseSmall
unconditionally for the same kind of reason (size is its budget) since before this.

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