fix: clangd hanging fixed

[neriousy]

closes #3598

what was the problem:

• concurrent requests tried to spawn clangd simultaneously, so the broken-set flipped true and left the workspace without a live client.
• when clangd was missing from PATH the downloader only looked for a hard-coded .zip, so newer .tar.xz releases (or cached installs with different naming) never bootstrapped.

what's been changed:

• guarded LSP boot with a spawning promise cache so identical server/root pairs reuse an in-flight spawn instead of racing.
• refreshed clangd discovery to reuse binaries in ~/.local/share/opencode/bin, scan prior clangd_* extractions, and fall back to GitHub only when needed.
• normalized release asset selection to accept either .zip or .tar.xz, reject empty downloads, and unzip/untar accordingly.
• rewired the installer to recreate a clangd symlink after every install so the runtime always launches via a stable path regardless of the versioned folder layout.

how to check if it works:

1. remove the clangd from your ~/.local/share/opencode/bin and path
2. create two files main.c and math_utils.h

main.c:

// #include "math_utils.h"
#include <stdio.h>

int main() {
    int a = 5, b = 3;
    int sum = add(a, b);
    int diff = subtract(a, b);

    printf("Sum: %d\n", sum);
    printf("Diff: %d\n", diff);
    return 0;
}

math_utils.h:

#ifndef MATH_UTILS_H
#define MATH_UTILS_H

int add(int a, int b);
int subtract(int a, int b);

#endif

ask opencode to fix the main.c file
it should download the LSP and create a symlink

neriousy@FILIP:~/programming/opencode$ ls ~/.local/share/opencode/bin/
clangd  clangd_21.1.0  fzf  gopls  rg  vscode-eslint