feat: vendor the public read contract, and give it its own source switch

The public surface — 17 of the 28 registered routes — reads from a 29KB
TypeScript fixture and never touches the backend. `TECH_LOG_STUDIO_SOURCE`
only ever switched the Studio gateways; `publicContent` was wired to the
static adapter unconditionally, so no configuration could make the public
site show published content. This is the first half of closing that: the
contract and the switch, with the adapter still to come.

The generator now vendors both canonical contracts instead of one. They are
independent — different services on different schedules — so each carries
its own digest and operation list, and updating one leaves the other's drift
gate quiet.

`TECH_LOG_PUBLIC_SOURCE` is deliberately a second flag rather than a rename
of the Studio one. The combination that matters right now is exactly the one
a single flag cannot express: the authoring backend is live while the public
read API does not exist yet. production stays on MOCK for that reason —
pointing it at HTTP today would empty the live site — and moves when the
backend serves /api/v1/public.

Also records the compatibility evidence the registry gate wanted for the
Studio access change in fff5e6f. That gate has been failing since, which is
on me: the change was real and breaking, and it shipped without the note
explaining that route ids and schemas are untouched and only the access
classification moves.

config/runtime/development.json

@@ -14,6 +14,7 @@
     "OFFLINE_COMMANDS": "DEFAULT"
   },
   "TECH_LOG_STUDIO_SOURCE": "HTTP",
+  "TECH_LOG_PUBLIC_SOURCE": "HTTP",
   "FEATURE_OVERRIDES": {
     "reference-feature": "DEFAULT"
   }

config/runtime/local.json

@@ -14,6 +14,7 @@
     "OFFLINE_COMMANDS": "DEFAULT"
   },
   "TECH_LOG_STUDIO_SOURCE": "MOCK",
+  "TECH_LOG_PUBLIC_SOURCE": "MOCK",
   "FEATURE_OVERRIDES": {
     "reference-feature": "DEFAULT"
   }

config/runtime/production.json

@@ -15,6 +15,7 @@
     "OFFLINE_COMMANDS": "DEFAULT"
   },
   "TECH_LOG_STUDIO_SOURCE": "HTTP",
+  "TECH_LOG_PUBLIC_SOURCE": "MOCK",
   "FEATURE_OVERRIDES": {
     "reference-feature": "DEFAULT"
   }

config/runtime/staging.json

@@ -15,6 +15,7 @@
     "OFFLINE_COMMANDS": "DEFAULT"
   },
   "TECH_LOG_STUDIO_SOURCE": "HTTP",
+  "TECH_LOG_PUBLIC_SOURCE": "HTTP",
   "FEATURE_OVERRIDES": {
     "reference-feature": "DEFAULT"
   }