sketch(520): typed pipeline state machine — bare and statig versions

Two parallel scratch experiments under server/examples/ exploring the
typed Rust state machine that should replace huskies's current
stringly-typed CRDT representation (story 520).

  - pipeline_state_sketch_bare.rs   — hand-rolled, plain enums + match
  - pipeline_state_sketch_statig.rs — using the statig crate

Both sketches:
  - Define the same Stage enum (Backlog, Coding, Qa, Merge, Done, Archived)
  - Define ArchiveReason (subsumes refactor 436's blocked/merge_failure/review_hold)
  - Define ExecutionState (per-node, separate from synced Stage) — bare only
  - Define PipelineEvent and the valid transitions
  - Make bug 519 unrepresentable: Stage::Merge requires NonZeroU32 commits_ahead
  - Make bug 502 unrepresentable: Coder agents can't be assigned to Merge state
  - Have happy-path tests, retry-loop tests, and invalid-transition tests

Differences:
  - Bare uses pure pattern matching, no framework. ~720 lines.
  - Statig uses #[state_machine] proc macro and gets free hierarchical
    states via the `active` superstate that factors out the cross-cutting
    Block / ReviewHold / Abandon / Supersede transitions across the four
    active stages. ~440 lines, 11 passing tests.

Run with:
  cargo run  --example pipeline_state_sketch_bare   -p huskies
  cargo run  --example pipeline_state_sketch_statig -p huskies
  cargo test --example pipeline_state_sketch_bare   -p huskies
  cargo test --example pipeline_state_sketch_statig -p huskies

Adds statig 0.3 as a dev-dependency in server/Cargo.toml. Cargo.lock
updated to include statig + statig-macro and their transitive deps.

Not wired into the main codebase. Once we agree on which version to
adopt, story 520 promotes the chosen sketch into a real
server/src/pipeline_state.rs module.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -2331,6 +2331,7 @@ dependencies = [
  "serde_yaml",
  "sha2 0.11.0",
  "sqlx",
+ "statig",
  "strip-ansi-escapes",
  "tempfile",
  "tokio",
@@ -4097,6 +4098,30 @@ dependencies = [
  "toml_edit 0.25.10+spec-1.1.0",
 ]
 
+[[package]]
+name = "proc-macro-error"
+version = "1.0.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "da25490ff9892aab3fcf7c36f08cfb902dd3e71ca0f9f9517bea02a73a5ce38c"
+dependencies = [
+ "proc-macro-error-attr",
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
+ "version_check",
+]
+
+[[package]]
+name = "proc-macro-error-attr"
+version = "1.0.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a1be40180e52ecc98ad80b184934baf3d0d29f979574e439af5a55274b35f869"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "version_check",
+]
+
 [[package]]
 name = "proc-macro-error-attr2"
 version = "2.0.0"
@@ -5722,6 +5747,27 @@ version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a2eb9349b6444b326872e140eb1cf5e7c522154d69e7a0ffb0fb81c06b37543f"
 
+[[package]]
+name = "statig"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "42c467cc59664639bf70b8225b1b4a9c30d926f3e010c29e804bf940d618c663"
+dependencies = [
+ "statig_macro",
+]
+
+[[package]]
+name = "statig_macro"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bf4c61563b68df6e452ceece3fba1329c8c6a5d348fe17b0778fada28bc95fde"
+dependencies = [
+ "proc-macro-error",
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
+]
+
 [[package]]
 name = "string_cache"
 version = "0.8.9"