refactor: Rename threshold to reconstruction_threshold across node providers

[SimonRastikian]

Improves readability of the code

[claude]

PR title type suggestion: This PR renames fields/variables across the node providers codebase, so the type prefix should be refactor:.

Suggested title: refactor: rename threshold to reconstruction_threshold across node providers

[claude]

Pull request overview

Mechanical rename of threshold → reconstruction_threshold across the node providers (ecdsa, robust_ecdsa, eddsa, ckd, verify_foreign_tx), the SignatureProvider trait, key_events, the coordinator, and test fixtures. Targets the 3164-adding-ReconstructionThreshold-in-node follow-up branch and re-applies the rename portion that was previously reverted.

No semantic changes. Local bindings, struct fields, function parameters, and the HashMap<DomainId, ReconstructionThreshold> plumbing are renamed; calls into the external threshold_signatures crate keep their library-defined field name (PresignArguments { threshold: ... }).

Changes:

• Rename threshold / new_threshold / old_threshold parameters on SignatureProvider::run_key_generation_client / run_key_resharing_client and their implementations across all providers.
• Rename threshold field to reconstruction_threshold on KeyGenerationComputation, KeyResharingComputation, SignComputation, FollowerSignComputation, PresignComputation, FollowerPresignComputation, ManyTripleGenerationComputation, FollowerManyTripleGenerationComputation for ECDSA / robust-ECDSA / EdDSA / CKD.
• Rename thresholds constructor argument to reconstruction_thresholds on EcdsaSignatureProvider::new, RobustEcdsaSignatureProvider::new, EddsaSignatureProvider::new, CKDProvider::new.
• Rename domain_to_threshold HashMap in coordinator.rs and the local binding in key_events.rs.
• Touch test fixture gen_four_participants and several #[cfg(test)] constructors to use the new name.

Reviewed changes

Per-file summary

File	Description
crates/node/src/assets/test_utils.rs	Rename local binding in gen_four_participants.
crates/node/src/coordinator.rs	Rename domain_to_threshold → domain_to_reconstruction_threshold.
crates/node/src/key_events.rs	Rename locals in keygen_computation_inner / resharing_computation_inner.
crates/node/src/providers.rs	Rename trait method parameters on SignatureProvider.
crates/node/src/providers/ckd.rs	Rename constructor arg and trait-method params.
crates/node/src/providers/ckd/key_generation.rs	Rename KeyGenerationComputation::threshold field.
crates/node/src/providers/ckd/key_resharing.rs	Rename KeyResharingComputation::{threshold, old_threshold} fields.
crates/node/src/providers/ckd/sign.rs	Rename local in CKD sign client.
crates/node/src/providers/ecdsa.rs	Rename constructor arg, trait params, and locals in background jobs / triple_store_for_t.
crates/node/src/providers/ecdsa/key_generation.rs	Rename field on KeyGenerationComputation.
crates/node/src/providers/ecdsa/key_resharing.rs	Rename fields on KeyResharingComputation.
crates/node/src/providers/ecdsa/presign.rs	Rename fields on PresignComputation / FollowerPresignComputation and locals.
crates/node/src/providers/ecdsa/sign.rs	Rename fields on SignComputation / FollowerSignComputation and locals.
crates/node/src/providers/ecdsa/triple.rs	Rename TripleStorage::new param, ManyTripleGenerationComputation::threshold, and the follower variant.
crates/node/src/providers/eddsa.rs	Rename constructor arg and trait-method params.
crates/node/src/providers/eddsa/key_generation.rs	Rename field on KeyGenerationComputation.
crates/node/src/providers/eddsa/key_resharing.rs	Rename fields on KeyResharingComputation.
crates/node/src/providers/eddsa/sign.rs	Rename field on SignComputation and locals.
crates/node/src/providers/robust_ecdsa.rs	Rename constructor arg and trait-method params.
crates/node/src/providers/robust_ecdsa/presign.rs	Rename function param and compute_thresholds param.
crates/node/src/providers/verify_foreign_tx.rs	Rename _threshold placeholder params on stub trait impls.

Findings

No blocking issues — the diff is a faithful mechanical rename, behavior is unchanged, and the one remaining threshold: (in PresignArguments, crates/node/src/providers/ecdsa/presign.rs:255) correctly preserves the external library's field name.

Non-blocking (nits, follow-ups, suggestions):

• crates/node/src/providers/ecdsa.rs:140 — triple_store_for_t now takes reconstruction_threshold: ReconstructionThreshold, so the _for_t suffix is the last place that still uses the short name. Optional: rename to triple_store_for_reconstruction_threshold for consistency, or leave as-is if the abbreviation is preferred for the method name.
• crates/node/src/providers/ecdsa/presign.rs:189 — error message still reads "presign participant count ({}) does not match domain threshold t={}". Not wrong (it's user-facing prose referring to the protocol parameter t), but worth a glance if you want operator-visible strings to track the variable rename.
• crates/node/src/assets/test_utils.rs:45 — doc comment "Generates a 4-participant test fixture with threshold 3" still says "threshold"; harmless prose, but trivially aligns with the new vocabulary by saying "reconstruction threshold 3".

✅ Approved