feat(shader): screen_texture / screen_uv / noise_uv builtins + {vertex, fragment} shader assets (+ Water Overworld example) (#1548)

* feat(shader): Godot-style builtins — screen_texture / screen_uv / noise_uv

Inside a ShaderEffect fragment body (design contributed by the melonJS
editor team, mirroring Godot's hint_screen_texture / SCREEN_UV):

- `uniform sampler2D name : screen_texture;` — annotation parsed and
  stripped; the sampler is auto-wired to the renderer's shared
  toFrameTexture capture (live GPU-resident entry) and the engine
  refreshes the capture right before the effect draws: in drawImage's
  customShader block (scene behind the sprite) and in endPostEffect
  (camera FBO = the scene for camera chains, parent target for
  renderable chains). True BackBufferCopy semantics on every path.
  Optional `(repeat)` wrap honored under WebGL 2 (captures are NPOT).
- `screen_uv` — free varying, clip→0..1 screen position, computed in a
  deterministically-templated vertex variant (user source never regex-
  rewritten).
- `noise_uv` — free varying, frame-local 0..1 across the drawn object
  (undoes atlas packing, scaled to object pixels); fed per draw through
  the new @ignore ShaderEffect._setNoiseUVRect from the quad batchers'
  single-texture path and both blitTextures.

Back-compat: annotation was never valid GLSL; varyings activate only
when referenced and never when the body declares them itself; a body
using no builtins compiles byte-identical to before. New renderer
helper getSharedFrameTexture() gives the shared capture slot a stable
identity from effect-construction time.

8 new tests (shadereffect-builtins.spec.js): parse/wiring/introspection,
true screen sampling + orientation via two-tone backdrop + swizzle
readback, frame-local noise_uv on an atlas sub-frame (both axes),
no-capture-without-annotation (spy), clone, self-declared back-compat.
Full suite 4810 passed | 15 skipped.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019t8kP8vp58ZrvaD2FqJU6A

* feat(examples): Water Overworld — the editor team's demo scene, ported 1:1

Side-scroller overworld (GandalfHardcore free 32x32 pack) with the
water-refraction shader from the contributed build, verbatim: two
cellular NoiseTexture2d flow maps + screenTex : screen_texture +
screen_uv mirror + noise_uv flow — the canonical showcase of the new
shader builtins. TMX level, atlas strip-sliced animated props
(campfire, portal, cooking area), drifting clouds, playable character
(A/D walk, W jump, Shift run — their controller plugin replaced by
~25 lines of plain melonJS input/body code with the same tuning).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019t8kP8vp58ZrvaD2FqJU6A

* fix(examples): waterOverworld parity with the reference build

- water sprite uses the two regions verbatim (their settings semantics)
  instead of getAnimationSettings, whose bottom-align trim mis-placed
  the frame and bled the adjacent cloud5 atlas pixels (the white
  artifact on the left edge); the 2dWater art carries its own
  transparent sky band, matching the original placement
- clouds drift right and respawn off the left edge (ported from the
  demo's update user-code: speed 1+random, wrap at x 1040)
- handle video.init() returning false (CodeRabbit finding), alongside
  the existing thrown-error path

Verified side-by-side against the zip's build served statically —
scene, water surface line, reflection and drift now match; CodeRabbit
re-review clean (0 findings).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019t8kP8vp58ZrvaD2FqJU6A

* feat(examples): waterOverworld — debug panel + arrow-key controls

- register @melonjs/debug-plugin (press "S" to toggle), like the other
  examples and the original demo build
- arrow keys as movement aliases: ←/→ walk, ↑ jump (alongside A/D/W)
- controls hint updated

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019t8kP8vp58ZrvaD2FqJU6A

* test(shader): cover both endPostEffect screen-capture paths

The camera chain (capture from the still-bound camera FBO = the scene,
orientation preserved, exactly one toFrameTexture call) and the
multi-effect renderable chain (capture = the parent target behind the
object, not the object's own rendering) were the only untested branches
of the builtins feature — the example and prior tests only exercised
the sprite/customShader path. 10 tests total now.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019t8kP8vp58ZrvaD2FqJU6A

* feat(loader): {vertex, fragment} shader-asset pairs compile into raw GLShaders

A shader asset can now be a complete program pair — src: {vertex,
fragment} URLs (fetched in parallel) or inline via data: — compiled at
load time into a shared raw GLShader, returned by the same
loader.getShader(). This is the honest type for full programs (Mesh
custom shaders, renderer.customShader, custom batchers) and sidesteps
ShaderEffect's apply() wrapping entirely: no source rewriting, no
builtins injection — the user owns both stages. Same shared/clone/
unload lifecycle as fragment-body assets; WebGL-only (a pair loads as
null under Canvas, with a warning — no canvas analog for a raw
program; unload guards the null entry).

6 adversarial tests (shader-loader.spec.js), mutation-verified: drop
shared flag / swap vertex-fragment stages / remove the already-loaded
guard each kill exactly their matching test. Full suite 4818 | 15
skipped.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019t8kP8vp58ZrvaD2FqJU6A

* docs(changelog): note the {vertex, fragment} shader-asset pair form

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019t8kP8vp58ZrvaD2FqJU6A

* docs: drop third-party engine references from comments, docs, changelog

The builtins stand on their own names — comparisons to other engines'
equivalents removed from JSDoc, module comments, spec headers, the
example, and the 19.9.0 changelog entries (generic "back-buffer copy"
terminology kept where technically descriptive).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019t8kP8vp58ZrvaD2FqJU6A

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: obiot

packages/examples/public/assets/waterOverworld/image/Floor Tiles1.webp

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