deps: disable default features on x25519-dalek (consistency; drops ~40 KB of basepoint tables from downstreams)

[punarinta]

Summary

vodozemac already declares default-features = false on ed25519-dalek and curve25519-dalek, signaling it does not intend to require curve25519-dalek's precomputed-tables feature. But x25519-dalek is declared with its defaults, and x25519-dalek's default set includes precomputed-tables (default = ["alloc", "precomputed-tables", "zeroize"]).

Because Cargo unifies features across the whole dependency graph, this single declaration silently re-enables curve25519-dalek/precomputed-tables for the entire build — negating the default-features = false on the other two dalek crates, and forcing the large precomputed basepoint lookup tables into every downstream binary, with no way for a downstream to opt out (Cargo has no mechanism to disable a transitively-requested feature).

Change

-x25519-dalek = { version = "2.0.1", features = ["serde", "reusable_secrets", "static_secrets", "zeroize"] }
+x25519-dalek = { version = "2.0.1", default-features = false, features = ["alloc", "serde", "reusable_secrets", "static_secrets", "zeroize"] }

alloc is re-added explicitly so the only net change to the resolved feature set is the removal of precomputed-tables.

Affected symbols

Symbol	Size
curve25519_dalek::…::ED25519_BASEPOINT_TABLE_INNER_DOC_HIDDEN	~30 KB
curve25519_dalek::backend::vector::avx2::…::BASEPOINT_ODD_LOOKUP_TABLE	~10 KB

Measured impact

Measured on a downstream cdylib that links vodozemac (release: lto = true, opt-level = 3, codegen-units = 1, stripped):

before: 1,002,496 bytes
after:    965,240 bytes   (−37,256 bytes, −3.7%)

The downstream test suite (Olm session round-trips, pickling, KATs) passes unchanged — precomputed-tables is a pure size/speed trade with no behavioral effect.

Tradeoff

Disabling precomputed-tables removes the fixed-base scalar-multiplication speedup, which affects key generation (PublicKey::from(&StaticSecret)) and Ed25519 signing. It does not affect x25519 ECDH (variable-base — the hot Double Ratchet operation is unchanged). These fixed-base operations are not hot paths for typical messaging workloads, and this aligns vodozemac's actual behavior with what its ed25519-dalek/curve25519-dalek declarations already intend.

If you'd prefer to keep the speedup as the default, an alternative is to expose it as an opt-in feature (e.g. precomputed-tables = ["curve25519-dalek/precomputed-tables"]) so size-sensitive downstreams can leave it off. Happy to adjust the PR in whichever direction you prefer.