Reject or recover incomplete Linux environment entries instead of returning truncated values

[LunaStev]

Reproduced problem

env_get reads at most 32,768 bytes into its internal environment buffer. The Linux provider performs one read of /proc/self/environ. If that read ends in the middle of an entry, the high-level parser treats the buffer boundary as the end of a complete value and returns success.

With an environment containing only WAVE_AUDIT_VALUE set to 40,000 x characters, this program prints count=32751, despite a destination large enough for the full value:

import("std::env::environ")::{env_get};
fun main() -> i32 {
    var output: array<u8, 50000>;
    var count: i64 = env_get("WAVE_AUDIT_VALUE", &output[0], 50000);
    println("count={}", count);
    return 0;
}

After building it as long_env, run the binary with a controlled environment:

import subprocess
subprocess.run(["/absolute/path/to/long_env"],
               env={"WAVE_AUDIT_VALUE": "x" * 40000}, check=True)

Scope and acceptance

• Never return a successful value assembled from an unterminated partial environment entry.
• Support continuing/growing the read, or return an explicit capacity/read error rather than silently truncating or reporting a present key as absent.
• Handle keys after the initial 32 KiB and exact-boundary entries.
• Preserve the distinction between internal source truncation, caller destination-too-small and a genuinely missing key.
• Add controlled-environment regressions; tests must not inspect the developer's real environment.
• Keep Linux environment access through the existing raw OS path, with no libc binding workaround.

Start with std/env/environ.wave:67-104 and std/sys/linux/{amd64,arm64,riscv64,loong64}/env.wave. Related #427 covers general environment boundary tests; #423 and #424 cover presence checks and integer overflow. This issue is specifically the incomplete source-read bug.

Audited on canonical master 0c67f4cc0c3946cbf708c11ef79db927ec8f054e (same source tree as #502 head). Executable reproductions used Fedora Linux amd64, Wave 0.2.1-pre-beta-dev, LLVM 21.1.8.