Rename legacy staged identifiers to the staging naming

[aram356]

Summary

• Pure rename, no behavior change: internal identifiers still using the older "staged" naming now match the user-facing deploy --staging terminology.
• DeployStaged → DeployStaging (both enums), deploy_staged → deploy_staging, split_staged_passthrough → split_staging_passthrough, StagedPassthrough → StagingPassthrough, plus the test helpers and test names built from them; doc-comment back-references updated.

Changes

Crate / File	Change
crates/edgezero-adapter/src/registry.rs	AdapterAction::DeployStaged → DeployStaging
crates/edgezero-cli/src/adapter.rs	Action::DeployStaged → DeployStaging (Display string deploy --staging unchanged)
crates/edgezero-cli/src/lib.rs	Updated Action::DeployStaging call sites
crates/edgezero-adapter-fastly/src/cli.rs	deploy_staged → deploy_staging, split_staged_passthrough → split_staging_passthrough, StagedPassthrough → StagingPassthrough, test helpers and test names, doc back-references
crates/edgezero-adapter-{axum,cloudflare,spin}/src/cli.rs	Renamed variant in unsupported-action match arms

Closes

Closes #374

Test plan

• cargo test --workspace --all-targets (1421 passed, 0 failed)
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• cargo check --workspace --all-targets --features "fastly cloudflare spin"
• WASM builds: wasm32-wasip2 (Spin) via cargo check -p edgezero-adapter-spin --target wasm32-wasip2 --features spin; Fastly/Cloudflare targets covered by the feature check above and CI
• examples/app-demo workspace: cd examples/app-demo && cargo test --workspace --all-targets
• Docs build: cd docs && npm run lint && npm run format && npm run build
• Manual testing via edgezero serve --adapter axum
• Other: repo-wide grep confirms zero remaining matches for the old identifiers

Checklist

• Changes follow CLAUDE.md conventions
• No Tokio deps added to core or adapter crates
• Route params use {id} syntax (not :id)