requirements:
  - id: R1
    title: Mobile 2D overhead curling
    body: Players play curling in a mobile browser from a 2D overhead view.
  - id: R2
    title: Portrait sheet
    body: The sheet is shown in portrait.
  - id: R3
    title: Two teams, unlimited clients
    body: A room has two teams and an unlimited number of clients.
  - id: R4
    title: Team pick per device
    body: Each client picks Team 1 or Team 2 on that device.
  - id: R5
    title: Anyone on turn may throw
    body: Anyone on the team whose turn it is may throw.
  - id: R6
    title: Shareable room link
    body: Players join a room from a shareable link.
  - id: R7
    title: Create room from empty link
    body: If the link has no room, the client creates one and writes that room into the URL.
  - id: R8
    title: Ten ends, eight stones
    body: A match has 10 ends. Each team has 8 stones per end.
  - id: R9
    title: Extra ends until a lead
    body: Extra ends are played while the score is tied after 10 ends. The match ends when one team leads after an end.
  - id: R10
    title: Random hammer
    body: Hammer is random at the start. The team without hammer throws first in the end.
  - id: R11
    title: Hammer after score or blank
    body: The team that scores gives hammer away. A blank end keeps hammer.
  - id: R12
    title: Alternate throws
    body: After a throw, that team has one fewer stone and the other team throws.
  - id: R13
    title: Score when stones are gone
    body: When all throws have completed, the end is scored.
  - id: R14
    title: House only scores
    body: Only stones touching the house can score.
  - id: R15
    title: Count to opponent nearest
    body: The team closest to the tee scores one point for each of its stones nearer than the opponent's nearest stone.
  - id: R16
    title: Blank end is 0-0
    body: If no stone can score, the end is 0-0.
  - id: R17
    title: Shared live state
    body: Every client sees the current end, both scores, hammer, whose turn it is, stones remaining, stone positions, trajectories from all shots, and whether the match is over.
  - id: R18
    title: Winner on match end
    body: When the match ends, both scores are shown with the winning team.
  - id: R19
    title: Hack to broom
    body: A thrown stone starts at the hack and travels toward the broom. The player chooses initial speed and curl.
  - id: R20
    title: Clockwise is thrower's right
    body: Clockwise curl moves the stone to the thrower's right.
  - id: R21
    title: Stone bounce, no line bounce
    body: Stones bounce off other stones. Stones do not bounce back from a sideline or the backline.
  - id: R22
    title: Out of play rules
    body: A stone is out if it touches a sideline. A stone must fully cross the backline to be out. A stone is out if it never fully crosses the hog line.
  - id: R23
    title: Sheet markings
    body: The ice is a curling sheet with a house, hog line, sidelines, and a backline on the 12-foot ring.
  - id: R24
    title: Default house framing
    body: The default view frames the house, sidelines, and backline above the controls.
  - id: R25
    title: Pan to hog
    body: The player can pan up and down the sheet to the hog line.
  - id: R26
    title: Broom anywhere then throw
    body: On turn, the player drags to place the broom anywhere on the sheet, then throws.
  - id: R27
    title: Discrete throw weights
    body: The player sets throw weight on a discrete scale of draw weights 1 through 10, then named takeouts hack, board, control, normal, and peel.
  - id: R28
    title: Curl direction
    body: The player sets curl clockwise or counterclockwise. Clockwise is the default.
  - id: R29
    title: Throw only on turn while still
    body: Throw controls work only on that team's turn and only while stones are not moving.
  - id: R30
    title: Stones, hammer, scoreboard HUD
    body: The HUD shows eight remaining-stone markers per team, a hammer marker, and a scoreboard with each end and a total.
  - id: R31
    title: End overlay
    body: After the stones stop and the end is scored, an end-result overlay appears. The player dismisses it.
  - id: R32
    title: Shared motion, hide out stones
    body: All clients see the same stone motion. A stone that leaves play is hidden when it leaves the sheet.
  - id: R33
    title: Copyable errors
    body: Errors and the final result appear on the page as copyable text, not as a blocking alert.
  - id: R34
    title: No pinch-zoom, remember team
    body: Pinch-zoom is off. The client remembers the last team choice.
choices:
  - id: C1
    title: Stack
    body: Backend is Rust, Axum, WebSocket, Rapier2D, server-authoritative at 120 Hz. Frontend is TypeScript, Vite, Canvas2D, portrait-first touch.
  - id: C2
    title: Team colours
    body: Team 1 is red. Team 2 is yellow.
  - id: C3
    title: Wire protocol
    body: WebSocket JSON with snake_case tags. game_state carries totals, hammer, turn_team, per-end scoreboard, stones_remaining, and stones. Trajectories sample at 40 Hz from the 120 Hz sim. No room-full limit. No end_scored message.
  - id: C4
    title: Physics how
    body: Ice friction is a server µ(v) table shared by linear and angular damping. Curl uses initial |ω| = 5 rot / 14 s with clockwise to the thrower's right. Stone-stone restitution is 0.9. Sidelines and backline are not colliders; out-of-play is applied after sim. Stone ids are { team, n } with n 1 through 8. Lengths use FEET_TO_METERS = 0.3048.
  - id: C5
    title: Weight mapping
    body: Discrete UI weights map to calibrated open-ice stop speeds in SPEED_TABLE. The server takes velocity and curl, not a local friction scalar.
  - id: C6
    title: Out of scope
    body: No accounts, persistence, anti-cheat, replay log, turn timer, or sweeping.