docs: use --debug-symbols false for dx release builds

#25 in Riparion/riparion-retro — merged 2026-06-11

What

Adds --debug-symbols false --keep-names true to the dx build --release commands documented across the READMEs, and adds a just build <game> recipe that uses the same flags.

Why

dx build --release (web) currently logs:

ERROR wasm-opt failed with status code signal: 6 (SIGABRT)
compile unit size was incorrect (this may be an unsupported version of DWARF)

on every project, then "succeeds" by silently shipping the un-optimized wasm.

Root cause: dx 0.7.9 defaults --debug-symbols to true even for release, so rustc emits DWARF 5 (Rust 1.96) into the wasm. dx's pinned binaryen (version_127) can't parse DWARF 5 and SIGABRTs while reading it, so wasm-opt never runs (Binaryen issue #6391). lto = true aggravates it.

Building with --debug-symbols false keeps the DWARF out so wasm-opt runs — meaningfully smaller bundles — and --keep-names true preserves readable panic backtraces via the name section.

The matching generator-side fix (so newly-discovered apps inherit the flags) is a separate PR in riparion-cms (DX_TEMPLATE). Existing QA app build scripts were patched directly.

Scope

Docs + justfile only — no crate source changed, so no version bumps.

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