feat(serde-feather): implement phase 3 named-struct derive MVP (#187)

## Summary
- implement Phase 3 MVP derive support for `serde-feather`
- stabilize and expose `FeatherSerialize` / `FeatherDeserialize` behind
the `derive` feature
- enforce `derive` => `std`, and keep no-std derive out of scope for
this phase
- add compile-time validation for unsupported inputs (enums, tuple
structs, generics, unsupported serde attrs)
- add runtime + compile-fail tests for MVP contract coverage
- update `docs/project-serde-feather.md` and `crates/AGENTS.md` to align
contracts with implementation

## Implementation details
- runtime crate:
  - re-export stable derive macros
  - add compile-time guard message for invalid feature configuration
- macro crate:
  - add `#[proc_macro_derive(FeatherSerialize, attributes(serde))]`
  - add `#[proc_macro_derive(FeatherDeserialize, attributes(serde))]`
  - support only non-generic named structs
  - support attrs:
    - container: `rename`
- field: `rename`, `default`, `skip`, `skip_serializing`,
`skip_deserializing`
  - ignore unknown fields during deserialization
  - resolve crate path robustly via `proc-macro-crate`

## Validation
- `cargo fmt --all`
- `cargo check -p serde-feather`
- `cargo check -p serde-feather-macros`
- `cargo check -p serde-feather --features derive`
- `cargo check -p serde-feather --no-default-features`
- `cargo check -p serde-feather --no-default-features --features derive`
- `cargo test -p serde-feather -p serde-feather-macros`
- `cargo test -p serde-feather --features derive`
- `cargo test`

Author: kdy1

Cargo.lock

@@ -41,7 +41,7 @@
 
 - Keep `serde-feather` as the runtime-facing crate and `serde-feather-macros` as the proc-macro crate.
 - Keep binary-size-first defaults: minimal default features and no convenience dependencies by default.
-- Do not stabilize public derive macro identifiers before they are documented in `docs/project-serde-feather.md`.
+- Keep stable derive macro identifiers (`FeatherSerialize`, `FeatherDeserialize`) aligned with `docs/project-serde-feather.md`.
 
 ### dexdex-Specific Rules
 

crates/AGENTS.md

@@ -10,6 +10,7 @@ publish = false
 proc-macro = true
 
 [dependencies]
+proc-macro-crate = "3.3.0"
 proc-macro2 = "1.0.94"
 quote = "1.0.39"
 syn = { version = "2.0.100", features = ["full"] }