docs: add EXTENSIONS.md describing the WgslExtension feature

[schell]

Documents the existing WgslExtension trait and the
#[wgsl(extensions = [...])] macro attribute at the repo root in a new
EXTENSIONS.md.

This is pure documentation; no source or test changes are included. The
trait and macro attribute are already implemented and stable — the file
consolidates what was previously only available via the rustdoc on
crates/wgsl-rs/src/extension.rs and the DEVLOG entries from 2026-05-15
and 2026-05-29.

What it covers

• The WgslExtension trait (single required method, modify_ir)
• The extensions = [...] macro attribute syntax
• The 4-step pipeline (build IR → extensions → type substitution →
  render) with the generated code shown
• Working with ir::Module: item kinds, the preserved Rust attribute
  system (ir::Attribute, not rendered into WGSL)
• Three worked examples: no-op, SlabItemExt derive-driven codegen,
  and appending a const
• Why extensions exist and the two design decisions they encode
• The wgsl-rs-layout crate as a real-world consumer
• Constraints and pitfalls (run order, substitution timing, attribute
  non-rendering, scope visibility, rejection of non-WgslExtension
  types, etc.)
• Cross-references to source files, trybuild tests, and DEVLOG
  anchors

Verification

• cargo fmt --check passes
• All internal source/test links resolve against the current tree
• All cited line numbers (extension.rs:51-58,
  macros/src/lib.rs:105-113,479-495, ir/types.rs:9-24,
  DEVLOG 2026-05-15 / 2026-05-29 anchors) verified against the actual
  files

AI disclosure

Per AGENTS.md, this commit was generated with AI
assistance. The author is disclosed as Schell Carl Scivally with minimax-m3 <efsubenovex@gmail.com>.

[Copilot]

Pull request overview

Adds a new root-level document (EXTENSIONS.md) that consolidates how downstream crates can extend wgsl-rs by implementing WgslExtension and wiring extensions into #[wgsl(extensions = [...])], including pipeline ordering, IR details, and worked examples.

Changes:

• Documented the WgslExtension trait contract and where it lives in the codebase.
• Documented the extensions = [...] macro attribute syntax and execution ordering.
• Added guidance/examples for inspecting/modifying ir::Module, including preserved Rust attributes and common pitfalls.

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