refactor(#2772): enumflags2 for the edge set; drop re-export shims

Review follow-ups on #2785.

`DepWhen` wraps `BitFlags<TerminalState>` instead of a hand-rolled `u8`,
so the bit manipulation belongs to the library and `TerminalState` gains
its flag value from `#[bitflags]` rather than a `bit()` match anyone
could get wrong. `of`/`accepts`/`is_empty` become one-liners over it.

Serialization is written out by hand rather than derived: clippy's
`unsafe_derive_deserialize` fires on deriving over a type with unsafe
internals, and the honest fix is to say what the wire form is. It is now
the list of accepted outcomes — `["done","failed"]` — which reads better
than a bitmask and survives the bits being renumbered.

Also drops the `pub use` re-export of `DepWhen` / `TerminalState` from
hive-c0re's `model`. It existed so that `use super::model::…` kept
compiling, which is a shim for one consumer's convenience; the sites
import from `hive_jobq` directly now.

And removes comments narrating what the code used to be. Git holds that.

Cargo.lock

@@ -1116,6 +1116,27 @@ dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "enumflags2"
+version = "0.7.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1027f7680c853e056ebcec683615fb6fbbc07dbaa13b4d5d9442b146ded4ecef"
+dependencies = [
+ "enumflags2_derive",
+ "serde",
+]
+
+[[package]]
+name = "enumflags2_derive"
+version = "0.7.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "67c78a4d8fdf9953a5c9d458f9efe940fd97a0cab0941c075a813ac594733827"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.119",
+]
+
 [[package]]
 name = "equivalent"
 version = "1.0.2"
@@ -1705,6 +1726,7 @@ name = "hive-jobq"
 version = "0.1.0"
 dependencies = [
  "chrono",
+ "enumflags2",
  "serde",
  "serde_json",
  "thiserror 2.0.18",