proxy: fix EVALSHA_RO fallback, guard script cache on primary error, bounded eviction

[Copilot]

Three bugs in the proxy-side script cache introduced in the previous commit.

EVALSHA_RO fell back to EVAL instead of EVAL_RO

Read-only semantics were silently dropped on secondary replay. evalFallbackArgs now maps EVALSHA_RO → EVAL_RO and EVALSHA → EVAL.

Script cache mutated on primary failure

rememberScript (including clearScripts for SCRIPT FLUSH) was called before checking the primary's error. A failed SCRIPT FLUSH would clear the proxy cache while the primary still held its scripts, breaking subsequent EVALSHA → EVAL fallbacks. Moved rememberScript to after the error guard.

Unbounded script cache

The scripts map[string]string had no size limit. Added maxScriptCacheSize = 512 with FIFO eviction tracked via an insertion-order scriptOrder []string slice—no new dependencies required.

// storeScript now evicts oldest entry at capacity
if len(d.scripts) >= maxScriptCacheSize {
    oldest := d.scriptOrder[0]
    d.scriptOrder = d.scriptOrder[1:]
    delete(d.scripts, oldest)
}

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