Add 3D clamshell phone dialer spike + persistent-board design

A diegetic 90s handset rendered as a small three.js scene in a fixed 1:2
portrait stage (visual spike at /?phone=1, three.js lazy-loaded so the
board bundle is unchanged). Blocky flat-shaded clamshell with glowing
keys, a flip-open animation with an intro camera orbit that settles
head-on before the DOM screen appears, a hinge barrel on the pivot axis,
and a chubby antenna. Pressable 3D keypad (raycast + keyboard) with DTMF
tones drives a dialer: connect / voicemail / SIT "unobtainable", against
a stub number->node directory with flag-gated node enablement.

The screen UI is real DOM positioned in percent of the stage, kept
pixel-exact by the head-on ortho camera.

Also documents the persistent-board flow model (docs/persistent-boards.md):
board_key reuse across level nodes, present-but-hidden flag-gated exhibits
with live arrival reveals, reset/new-game semantics, and A/B/M citation codes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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