Use TLS record limit for REALITY target buffer

[M13-Perfect]

Related to XTLS/Xray-core#6356.

Summary

REALITY currently uses a hardcoded 8192-byte buffer size for target TLS records.

This patch replaces the hardcoded value with the existing TLS record bound:

recordHeaderLen + maxCiphertextTLS13

This keeps the existing size variable and all downstream logic unchanged,
while deriving the bound from existing TLS constants instead of using an
arbitrary value.

Rationale

The linked Xray-core issue shows that www.microsoft.com can return a TLS
Certificate record with total length 8273 bytes, which exceeds the previous
8192-byte limit.

maxCiphertextTLS13 is already defined as 16384 + 256, and recordHeaderLen
accounts for the TLS record header. Using these existing constants keeps the
buffer bounded and aligned with the TLS 1.3 record boundary.

Relation to #33

This is an alternative implementation to #33. Instead of using 17 * 1024,
this patch reuses the existing TLS constants and keeps the change to a single
line.

Validation
Ran gofmt -w tls.go
Ran git diff --check
Verified in Docker

[M13-Perfect]

Hi maintainers, this is my first contribution to REALITY.

I kept the change intentionally small by reusing the existing TLS constants instead of introducing another literal buffer size.

If you prefer a dedicated constant name, I am happy to adjust the patch.