feat: encrypt stored records with AES-256-GCM

[DavidCozens]

Replace the HMAC policy with authenticated encryption. Tamper-evidence establishes that a stored
record was not altered; it does nothing to stop anyone reading it. AES-256-GCM encrypts the body,
authenticates the record header as associated data, and puts the nonce and tag in the trailer.

struct SolidSyslogMbedTlsAesGcmPolicyConfig gcmConfig = {.GetKey = SyslogStoreKey, .Rng = rng};

.SecurityPolicy = SolidSyslogMbedTlsAesGcmPolicy_Create(&gcmConfig),

GCM needs a fresh nonce per record and mbedTLS has no context-free RNG, so the policy takes the
device's DRBG as well as the key. That is the only wiring difference from the HMAC policy.

The store key does not change. Its name states what it protects rather than which algorithm protects
it, so escalating the policy needs no new key provisioned.

These are separate decisions and the second does not follow from the first. A device that only needs
to prove records were not altered can stop at the HMAC.

The pipeline element now derives both of its values from what the device holds, and each falls back
to the weakest honest answer when the credential behind it is missing:

s_sd[3] = SyslogPipelineSd_Init(
    ((clientChain != NULL) && (clientKey != NULL)) ? "mtls" : "tls", (rng != NULL) ? "aes-256-gcm" : "none"
);

... [logPipeline@32473 transport="mtls" atRest="aes-256-gcm"] device started

When you need it. If a disk that leaves the device would give something away — records naming
users, addresses, process values, or anything else you would not publish.

[DavidCozens]

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