docs: add WASM backend design document

Adds a Tier 1 design document for a WebAssembly backend covering the

opcodes currently implemented and tested in the IR layer. Branching,

control flow, host calls, and Tier 2 optimization are out of scope per

direction from @GloriousTacoo and will be added when the IR side

stabilizes for them.

The design follows PROGRAMMING_RULES.md: compile-time backend selection

via static-inline emit functions, cursor-based iteration over the

instruction array, single-pass emission with locals-count patching to

avoid a second cache pass, and read-only fields on the IR block

accessed directly rather than hoisted into locals.

Six open questions are collected in §16 for resolution in review before

the §12 instruction mapping table is finalized.

Signed-off-by: Proxy Alt <proxy-alt@proxy-alt.dev>

Author: Proxy-alt