Move chunked_config to edgezero-core and export it

[prk-Jr]

Summary

• Export the Fastly chunk-pointer writer/resolver (chunked_config) by moving it from edgezero-adapter-fastly (where it was pub(crate)) into edgezero-core, next to its only dependency blob_envelope.
• The functions are pure — all I/O goes through caller-supplied callbacks — so downstream hosts that read Fastly-shaped config stores via their own store abstractions can reuse the wire format (pointer schema, per-chunk len + sha verification, envelope reassembly) instead of re-implementing it. trusted-server carries a ~150-line copy of the resolver today because it must classify chunk read failures (retryable → 503) separately from verification failures (terminal → 500) at its own error boundary; the callback-based resolver expresses that, the transparent FastlyConfigStore::get path cannot.
• No behavior change; no wire-format change; edgezero-adapter-fastly keeps compiling unchanged via a pub(crate) re-export.

Changes

Crate / File	Change
crates/edgezero-core/src/chunked_config.rs	Module moved here from edgezero-adapter-fastly; API made pub (+#[inline] per strict-clippy); cli-feature cfg gates dropped (core has no cli feature; unused pure fns strip at link); module doc explains why it lives in core; all 13 unit tests move with it
crates/edgezero-core/src/lib.rs	pub mod chunked_config;
crates/edgezero-adapter-fastly/src/lib.rs	pub(crate) mod chunked_config; → pub(crate) use edgezero_core::chunked_config; gated any(cli, fastly, test), placed in the import group per arbitrary_source_item_ordering

Closes

n/a — issue linking skipped by request.

Test plan

• cargo test --workspace --all-targets
• 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-wasip1 (Fastly) / wasm32-wasip2 (Spin) / wasm32-unknown-unknown (Cloudflare)
• 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: downstream proof — trusted-server patched to this branch with its local resolver replaced by edgezero_core::chunked_config::resolve_fastly_config_value; full suite green (1475 core + 144 adapter tests) including the 503-vs-500 classification tests that motivated the local copy

Checklist

• Changes follow CLAUDE.md conventions
• No Tokio deps added to core or adapter crates
• Route params use {id} syntax (not :id) — n/a, no routing changes
• Types imported from edgezero_core (not http crate)
• Store wiring goes through KvRegistry / ConfigRegistry / SecretRegistry — n/a, no store wiring changes
• New code has tests (13 existing unit tests move with the module)
• No secrets or credentials committed

[aram356]

We should not have adapter specific code in the core. Maybe we can do this differently