fix: add bounded Node regex fallback search

[PracticalSwan]

Summary

• add regex support to the Node search fallback when ripgrep is unavailable
• execute JavaScript regex matching in an interruptible worker with a 5s ceiling
• retain existing file/result/scan bounds and reject oversized/invalid patterns cleanly
• decode ripgrep stdout/stderr with streaming UTF-8 decoders so chunk boundaries cannot corrupt JSON/text
• expose lightweight search capability metadata through server_config

Scope

This is intentionally narrower than issue #33: it does not add ffgrep/CodeGraph or a new indexing subsystem. It only restores bounded regex capability to the existing fallback without blocking the MCP server on pathological expressions.

It also does not duplicate open PR #115's Windows Bash/search-runtime diagnostics.

Verification

• clean branch based on upstream main (587f7fd)
• npm ci --ignore-scripts — 0 vulnerabilities
• npm run build
• focused runNodeFallbackSearchLimitStress() harness — passed
• git diff --check

The complete upstream stress file additionally reaches a pre-existing Windows Bash adapter failure (/bin/bash cannot execute a literal C:\...\node.exe command), which is unrelated to this search change and is covered by open PR #115.