fix(npm): install a runnable binary on every Linux arch and libc

[codyde]

npm i -g @railway/cli is broken on a large share of Linux hosts.

postinstall.js resolved the download target via @napi-rs/triples, which always answers *-unknown-linux-gnu for Linux. We only publish a gnu build for x86_64:

host	today
linux/arm64, arm, ia32	install fails — Failed fetching the binary: Not Found
linux/x64 on musl	installs, then won't run — Dynamic loader not found: /lib64/ld-linux-x86-64.so.2

That covers Graviton/Ampere, ARM CI runners, Docker on Apple Silicon, Raspberry Pi, and Alpine.

Changes

• explicit platform/arch → triple map matching exactly what release.yml builds, with Linux on the static musl builds — the choice install.sh already makes deliberately ("use the statically compiled musl bins on linux to avoid linking issues")
• unsupported platforms name what is supported instead of surfacing a bare 404
• drop @napi-rs/triples
• bin/railway.js no longer flattens every failure to exit 1. This is why agents.railway.com install fails silently in several common cases (segfault, set -e + whoami, sh vs bash) #990 reported "exits with code 1, no error message" for what was actually a crash.

child	before	after
exit 2 / 42 / 255	1 / 1 / 1	2 / 42 / 255
SIGSEGV / SIGINT	1 / 1	139 / 130
missing binary	1, no output	127 + how to fix

Verification — real npm i -g of the packed tarball on linux/amd64 and linux/arm64 × glibc (node:24) and musl (node:24-alpine), plus natively on darwin/arm64. All four Linux combinations failed before this change. Every mapped target confirmed to exist as a release asset.

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