docs: SECURITY.md bullet 3 overclaims stack_height on every auth path

[onspeedhp]

From the security review (docs/security-review-2026-04-21.md NBP-1).

Problem: SECURITY.md line 40 says "CPI stack_height anti-reentrancy check on every authenticated path." The Ed25519 authenticator (program/src/auth/ed25519.rs) does not check stack_height — by design, since Ed25519 relies on Solana's native tx-level signature verification + blockhash freshness.

Fix: tighten the wording to:

CPI stack_height anti-reentrancy check on every Secp256r1/session-authenticated path. Ed25519 relies on Solana's native tx-level signature verification.

Severity: low (doc-only).