fix(web): make wasm builds reproducible so the freshness gate passes

The web-wasm-freshness gate (#93) false-failed because the Bevy wasm build
baked in machine-specific absolute source paths — the cargo registry
(/home/<user>/.cargo/registry/...), the rustup std sources (~/.rustup/...),
and this checkout — so CI's rebuild (under /workspace, /root) never matched the
committed bytes even with identical pinned tool versions.

build_wasm.sh now exports CARGO_ENCODED_RUSTFLAGS with three --remap-path-prefix
rules (cargo home -> /cargo, rustup home -> /rustup, repo -> /build) so the
embedded paths are identical on any machine. It re-states the
getrandom_backend cfg because a *_RUSTFLAGS env var replaces (not merges with)
.cargo/config.toml's target rustflags.

Regenerated all four artifacts with the remap applied — verified zero
machine-specific paths remain (only canonical /cargo + /rustup prefixes), which
is the precondition for the gate's byte-for-byte rebuild-and-diff to match
across machines.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

build_wasm.sh

@@ -22,6 +22,22 @@ set -euo pipefail
 REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 OUT_DIR="$REPO_ROOT/solitaire_server/web/pkg"
 
+# Reproducible builds. The wasm artifacts otherwise bake in machine-specific
+# absolute source paths (the cargo registry, the rustup std sources, and this
+# checkout), so a rebuild on a different machine produces different bytes and
+# the CI freshness gate (rebuild-and-diff) false-positives. Remap all three
+# prefixes to fixed names so the output is byte-identical anywhere.
+#
+# We must use CARGO_ENCODED_RUSTFLAGS (not RUSTFLAGS) and re-state the
+# getrandom backend cfg here: a `*_RUSTFLAGS` env var *replaces* — does not
+# merge with — the `[target.wasm32-unknown-unknown] rustflags` in
+# .cargo/config.toml, so dropping that cfg would break the wasm getrandom build.
+# Keep this `--cfg` in sync with .cargo/config.toml.
+CARGO_HOME_DIR="${CARGO_HOME:-$HOME/.cargo}"
+RUSTUP_HOME_DIR="${RUSTUP_HOME:-$HOME/.rustup}"
+US=$'\x1f'   # unit separator: CARGO_ENCODED_RUSTFLAGS arg delimiter
+export CARGO_ENCODED_RUSTFLAGS="--cfg${US}getrandom_backend=\"wasm_js\"${US}--remap-path-prefix=${CARGO_HOME_DIR}=/cargo${US}--remap-path-prefix=${RUSTUP_HOME_DIR}=/rustup${US}--remap-path-prefix=${REPO_ROOT}=/build"
+
 if ! command -v wasm-pack &> /dev/null; then
     echo "error: wasm-pack not found." >&2
     echo "  Install with: cargo install wasm-pack" >&2