Add transformer mode: car unfolds into a shell-firing mech

[Makio64]

Depends on #1 (turtle shell / NPC racing). While #1 is open the diff shows both features; it will narrow automatically once #1 merges.

Summary

• Press T (or gamepad B/circle) — the car performs a staged mechanical transformation into a stationary mech that rapid-fires shells from shoulder cannons.
• Press T again to reverse. In robot mode the car can't drive — only aim and shoot.
• Built on the shell + NPC systems from PR Add throwable turtle shell and NPC racing opponents #1 (same Shell projectile, HitFX, contact listener, spin-on-hit).

The transformation

Staged choreography driven off a single progress value (0.9 s forward, 0.7 s reverse):

Phase	Action	FX
0.00–0.10 Anticipation	Car dips, wheels tuck in	Camera shake starts
0.10–0.30 Liftoff (overshoot)	Body rises, wheels hide, four chrome pods splay outward, underside plates explode out	Shockwave ring, steam puffs, CLUNK
0.30–0.55 Torso deploy	Spine extrudes, chest plate swings forward, back plate hinges up, blue core light ramps on	Cyan electric arcs between shoulder hinges, servo whine
0.55–0.75 Head + arms	Head slides up with overshoot, visor rotates open revealing red eye (flickers on), shoulder cannons unfold from folded-back to forward-ready	Steam puff, screen flash, CLACK
0.75–0.90 Pose	Cannons settle with spring oscillation, antenna pops up with overshoot, core flashes bright	Big shockwave, 18% white flash, gold star burst

Robot mode

• Physics body becomes MotionType.KINEMATIC, pinned at entry position, zero velocity.
• A / D: rotate torso yaw (turret swings, cannons follow).
• W / S: tilt cannon pitch ±14°.
• Space: rapid-fire shells at 0.12 s cooldown, max 12 active, alternating left/right cannon tips.
• Each shot: cannon recoils back 0.25 rad with spring-damped rebound; muzzle flash core + camera-facing flare at the tip; eye visor pulses brighter; cannon muzzle glows orange as heat accumulates (decays over 0.9 s).
• Idle torso bob when fully transformed.
• vehicle.stun() is a no-op during transform/transformer mode — robot is bullet-proof against its own ricochet.

File changes

• New js/Robot.js — procedural mech hierarchy (~25 meshes, 6 shared materials) + choreographed per-phase animation + getCannonTransform(side) helper.
• New js/TransformFX.js — pooled steam puffs, electric arcs (BufferGeometry Lines with jittered path), shockwave rings, and a camera-facing fullscreen white flash.
• New js/MuzzleFlash.js — pool of 8 cannon-tip bursts (orange core + billboarded flare).
• js/Vehicle.js — transform state machine, phase-cross FX triggers, kinematic swap, wheel hide / body raise during transform, turret input, camera-shake amplitude.
• js/Controls.js — rising-edge transform flag (KeyT + gamepad button 1).
• js/main.js — wire Robot + TransformFX + MuzzleFlash, route fire to cannon tips in robot mode, apply camera-shake post-offset.

Test plan

• Serve statically (python3 -m http.server 8000), open http://localhost:8000/.
• Normal drive works: shells, NPCs racing, HitFX on truck hits (all PR Add throwable turtle shell and NPC racing opponents #1 behaviour).
• Stop, press T: full staged sequence plays — car dips, body rises with overshoot, wheels hide, pods splay, spine extrudes, chest swings, back plate flips, head pops up, visor opens flickering, cannons unfold, final shockwave + flash + stars.
• Idle robot: torso bobs, blue core pulses, red eye glows steady.
• Hold Space in robot mode: alternating L/R cannons rapid-fire, recoil + muzzle flash each shot, muzzles heat up orange.
• A/D swings turret; shells fire along the new aim direction.
• W/S tilts cannons slightly.
• Press T again: reverse sequence runs faster, wheels re-appear, driving resumes normally.
• Transform mid-roll: car snaps to full stop, transforms, reverts — no drift after restore.
• Console clean, pool counts stable after multiple cycles.

🤖 Generated with Claude Code

[mrdoob]

Like link: https://raw.githack.com/Makio64/Starter-Kit-Racing/feature/transformer-mode/index.html