feat(fuzz): EncString parser fuzz harness (PRD §11.4) #71

Job	Run time
cargo-audit	3m 1s
cargo-deny	47s
headless build (no TUI / clipboard deps)	28s
rustfmt	16s
vault --version emits §13.2 attribution	37s
clippy	21s
test (macos-latest)	33s
test (ubuntu-latest)	30s
	6m 33s

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