feat(vault-api): post-quantum transport feature flag (X25519MLKEM768) #73

Job	Run time
cargo-deny	38s
test (ubuntu-latest)	43s
clippy	28s
cargo-audit	3m 7s
rustfmt	13s
headless build (no TUI / clipboard deps)	31s
test (macos-latest)	41s
vault --version emits §13.2 attribution	41s
	7m 2s

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