V2: resolve `application` vs `usecases` layer ambiguity

[ScriptedAlchemy]

Context

PR #707 has two foundational crates whose names imply the opposite of their actual architectural roles:

• tracedecay-application is the bottom-of-stack ports/contracts layer.
• tracedecay-usecases is the top-of-stack product application/use-case orchestration layer.

Both crates contain long module-level explanations because the one-shot split historically conflated them. V2 has not shipped, so we should remove the ambiguity rather than preserve it as permanent architecture documentation.

Goal

Make crate names match responsibilities and remove split-era semantic confusion.

Proposed direction

Converge toward a model such as:

• tracedecay-domain: pure domain/value contracts.
• tracedecay-contracts (or equivalent): ports + transport-neutral request/result contracts.
• tracedecay-application: product use-case orchestration.

This likely means renaming current tracedecay-application to a contracts/ports name and current tracedecay-usecases to tracedecay-application, but choose the minimal dependency-safe form after inventorying callers.

Work

• Inventory what belongs in domain vs ports/contracts vs orchestration.
• Rename/re-home the two layers so conventional names reflect actual responsibility.
• Remove split-era explanatory comments that exist only to explain misleading names.
• Repoint consumers directly; do not retain crate/module aliases solely for feat: TraceDecay V2 delivery (PR8–PR13) + daemon performance, memory, and reliability overhaul #707 compatibility.
• Preserve the useful dependency inversion boundary where ports have multiple or security-sensitive implementations.
• Do not collapse unrelated runtime/storage behavior into the contracts layer.

Acceptance criteria

• A new contributor can identify domain, contracts/ports and application orchestration without historical explanation.
• No two crates use application/usecases terminology for opposite layers.
• Dependency direction remains acyclic and explicit.
• No compatibility crate/alias is added to preserve the old names.
• Full feat: TraceDecay V2 delivery (PR8–PR13) + daemon performance, memory, and reliability overhaul #707 CI/journeys pass after the rename/cutover.

Related: #707.

[ScriptedAlchemy]

Implementation is in progress on sol/layer-names-1075: one-shot tracedecay-application → tracedecay-contracts and tracedecay-usecases → tracedecay-application, with no aliases. Workspace check/clippy and 1,410 focused crate tests are green before the latest #707 merge; post-merge verification is running before the child PR opens.

[ScriptedAlchemy]

Delivered by PR #1106 (sol/layer-names-1075, merged into the #707 branch at 1d90a66; GitHub did not auto-close because the PR targets the integration branch, not master). The former tracedecay-application (contracts only) is now tracedecay-contracts and the former tracedecay-usecases (implementation) is now tracedecay-application, with every consumer repointed (1,198 files) and canonical contract names restored in 5866a54. Verified on the merged tip a2db02c: cargo check --workspace --all-targets --locked and workspace clippy -D warnings clean.