fix: enforce the declared bash and jq floors when the file is sourced

[Martin Bens (SpiGAndromeda)]

lib/mcpserver_core.sh has declared bash 4.1+ and jq 1.7+ in its header line and in README.md §Requirements since 3.0.0. Nothing read either one.

What that cost

bash below 4.1 — the file still parses, so there was no early error. run_mcp_server died on its exec {_MCP_LIFELINE_FD}<> redirection with {_MCP_LIFELINE_FD}: not found and status 127, which an MCP host reports as nothing more than a server that failed to start. macOS ships 3.2.57 as /bin/bash, and both usual launch paths — a #!/usr/bin/env bash shebang, and a host manifest whose command is bash — resolve through PATH.

jq below 1.7 — nothing failed at all. The server answered every request while validate_tool_arguments returned the pre-3.0.0 verdict for a declared integer: reading a number back through tojson to catch a fraction the double rounded away depends on the literal preservation jq added in 1.7.

What the guard does

Sourcing checks bash, then jq's presence, then jq's version, and refuses with the requirement, the version found where there is one, and remediation for the platform — the package manager's command where one can be determined, a generic line otherwise.

The block sits above the file's own set -euo pipefail, so a refusal leaves the calling shell's options as it set them. It uses no construct newer than bash 3.2: a guard that cannot run on the version it rejects never gets the chance to speak. Diagnostics go to stderr — stdout carries the JSON-RPC stream, and no protocol error is constructible at that point anyway, since every envelope is built with jq -n.

A jq that is present but cannot run — wrong architecture, missing shared library — is named as such, with the path it resolved to, rather than reported as an outdated one.

For consumers

The floors are read at source time, so PATH has to select the intended jq and bash at that point. A server script that repaired PATH after sourcing moves that line above the source. An operator who cannot edit the script sets PATH in the host manifest's launch command, which also selects the bash the server runs under:

"command": "bash",
"args": ["-c", "export PATH=/opt/homebrew/bin:$PATH; exec /path/to/server.sh"]

Closes #21