feat(phase0): complete GC1 feasibility and GC2 shared format contracts

- Record user evidence for directory fallback to complete GC1 (10/10 checks passed)

- Expand Format Specification 0.1 to Revision 0.2 with normative shared contracts

- Author JSON Schema Draft-07 at schema/xzbt-0.1.schema.json

- Implement zero-dependency semantic validator at tools/validate-exhibit.mjs

- Create 12-case conformance fixture suite and automated test runner (12/12 passing)

- Update implementation status, verification gates, and gap closure decisions

- Add devlog entry covering stall recovery and Phase 0 current state

.labyricorn/devlog/phase-zero-feasibility-and-current-stall/contents.lr

@@ -60,3 +60,7 @@ After that evidence is obtained, Phase 0 still needs shared document/value/refer
 Phase 1, the runtime skeleton, has not started. Neither have Phases 2 through 9. There is no completed production schema, integrated audiovisual runtime, reference benchmark, or soak result. The two-hour development soak and eight-hour release soak remain later acceptance requirements.
 
 Work stopped in accordance with the request to report problems before advancing. The next step is to obtain the remaining conditional fallback evidence, complete the Phase 0 contracts and traces, and then review the gate before beginning Phase 1. The resolved Blob-module failure does not need to be repeated unless relevant code changes.
+
+## Development-server boundary
+
+A dependency-free loopback server was added later for possible source-level development work. It cannot count toward Phase 0: serving the probe changes the origin, security model, and module-loading conditions. The host process reached the probe successfully, but the in-app browser could not reach that host loopback listener and returned a refused-connection error. This did not change the Phase 0 boundary or start a later phase.