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>

docs/persistent-boards.md

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+# Persistent boards, gated exhibits, and citation codes
+
+Status: **proposed** (design locked, not implemented). Extends the story flow graph
+([story-graph.md](story-graph.md)) and the narrative layer
+([narrative-todo.md](narrative-todo.md)); interlocks with the Claim/Case Report
+work in [TODO.md](TODO.md) Milestone 5. Depends on the **flags / case-state**
+primitive (a per-playthrough key→value store) that the gates and dialogue effects
+also read and write.
+
+## Why
+
+A mystery is broken into short pieces so we can **meter how much evidence the
+player carries at once**. Interruptions — a phone call, a knock, a revealed note —
+should not throw the player off the board they are working on. So a single
+investigation scene is a **persistent, player-mutated board** that several graph
+nodes return to, and new evidence **arrives into that same board** when the story
+warrants it rather than being dumped up front or split across duplicated templates.
+
+Two capabilities, independent and both wanted:
+
+1. **A board that survives across nodes** — `[board] → [phone call] → [same board]`
+   returns the player to their exact arrangement and connections.
+2. **Flag-gated exhibits inside that board**, revealed with a diegetic "it arrived"
+   moment (reuses the existing document locator beam).
+
+## Board identity and reuse
+
+An authored level node **cannot** reference a clone id — the clone is created per
+playthrough at runtime. Instead:
+
+- A **level node** references a `level_template_version_id` **plus a `board_key`**
+  (a logical board slot, e.g. `"harbor-desk"`). `board_key` defaults to the
+  template if the author doesn't care about sharing.
+- A **playthrough** holds a map `board_key → level_id` (the mutable clone).
+- Entering a level node:
+  - if the playthrough already has a clone for that `board_key` → **reuse it**
+    (the player's mutated board, arrangement and connections intact);
+  - otherwise → **clone the template** and record `board_key → new level_id`.
+
+`board_key` is the thing that makes two nodes share a board. Two nodes on the same
+template with **different** keys get **separate** boards.
+
+## Gated exhibits
+
+- Every exhibit is **cloned in**, including gated ones; a gated exhibit carries a
+  **`reveal` condition over flags**. The player's saved board already contains the
+  note — it is simply inert until the flag flips.
+- **Visibility is computed server-side at load.** Given the playthrough's flags,
+  the play-mode payload includes only exhibits whose `reveal` condition is met;
+  hidden exhibits are stripped from the response (same discipline that already
+  hides authoring-only fields like `expectedPartyKind`). Players cannot peek
+  unrevealed evidence in the API.
+- **Gate the edges too.** Any `exhibit_connection` or event link whose endpoint is
+  a not-yet-revealed exhibit is hidden until that exhibit appears, so there are no
+  dangling references.
+
+## The arrival reveal
+
+- The board's effective content can change **while the player is standing on it** —
+  the phone is an always-available surface, so a call can set a flag mid-scene. So
+  reveal is **not load-only**: after any phone/dialogue interaction that sets flags,
+  re-evaluate board visibility and reveal live (the phone overlay refetches
+  visibility on hang-up).
+- Track a per-playthrough **`revealed-seen` set**. On (re)load or after an
+  interaction, any exhibit now visible but not yet in the set → play the arrival
+  animation (a sibling/reuse of the **document locator beam**), then add it to the
+  set. The flourish fires **once**, not on every reload.
+
+## Reset and New Game
+
+- **Reset = tidy the desk back to the delivered state.** Authored exhibits return
+  to their arrival positions and folders; the player's rearrangement is undone.
+  **Flags are untouched**, so anything a flag has already revealed is still
+  "arrived" and is restored at its authored position.
+- **New Game** re-clones the whole tree and is therefore the only thing that
+  **resets flags** and returns gated exhibits to hidden.
+
+**Open decision — player notes on reset.** Milestone 5's rule is "reset discards
+player-created claims." Current lean: reset also removes player-created exhibits
+(it restores the *delivered* state), and New Game is the only wipe of flags. The
+alternative is a gentler reset that re-files arrived exhibits but leaves the
+player's own notes in place. Settle before implementing.
+
+## Exhibit citation codes (A / B / M)
+
+A single running number breaks when players create many note exhibits, so codes
+live in fixed namespaces:
+
+- **Authored exhibits: group letter + index** — `A1, A2, …` for the initial
+  dossier, `B1, B2, …` for a batch that arrives later via a flag reveal, etc.
+  **The letter is the arrival group**, which ties straight into gated reveal: "new
+  evidence arrived" *is* the B-series lighting up. Indices are assigned **within
+  each group at freeze time** and frozen in the template, so authored codes never
+  shift, no matter what the player does.
+- **Player-created exhibits: a running `M` (miscellaneous) series** — `M1, M2,
+  M3…` in creation order, in their own namespace so player note-making cannot
+  disturb authored codes. M-numbering is per-level-instance and (given the reset
+  lean above) restarts only when player notes are cleared.
+- The author chooses an exhibit's **group**; the system owns the **numbering**.
+
+These codes are the **citation token everywhere** — board, luggage-tag Claims, the
+Case Report, and the LLM gate.
+
+## LLM gate contract
+
+- A gate can be **scripted to require specific authored codes** — e.g. "the report
+  must cite `A1` and `B2`, connected." That authored requirement list is the
+  deterministic backbone *under* the LLM, per the section-9 intent of scripting the
+  cognitive shim rather than trusting it blind.
+- If the player types the full report freehand, they must include the Exhibit codes;
+  the gate checks the required codes are present (and, later, that the reasoning
+  holds).
+
+## Relationship to fresh templates
+
+This **complements**, not replaces, new templates:
+
+- **Shared `board_key`** — interruptions *within* one investigation scene (calls, a
+  knock, a revealed note). Cleanly subsumes the earlier "note appears on the next
+  level" idea: the note is a gated exhibit on the *same* board that reveals after
+  the call.
+- **A fresh template** — the player genuinely moves to a new location/chapter with a
+  different board.
+
+## Implemented vs deferred
+
+Nothing here is built yet. New plumbing, smallest-first:
+
+1. **Flags** — the per-playthrough key→value store (shared with gates/dialogue).
+2. **`board_key` reuse** in level traversal + the playthrough `board_key → level_id`
+   map (clone-or-reuse).
+3. **`reveal` conditions on exhibits** + server-side play-mode visibility filtering
+   (exhibits and their edges).
+4. **`revealed-seen` set** + the arrival animation (locator-beam sibling), live
+   after flag changes.
+5. **A / B / M citation codes** — authored group + frozen index, player M-series;
+   surfaced on the board and threaded into Claims/report/LLM gate.
+
+Reset (desk-restore, flags-persist) and New Game (full re-clone) semantics as above.