Move input admission and Environment input reservations into sessions

[SaladDay]

Part of #1 (dissolve store).

Input validation, admission, steering, cancel requests and Environment input reservations move out of store and into sessions, which owns the pure decisions and procedures, and sessionpg, which loads and applies. sessions.AdmitInputs is now the one batch procedure for:

• public admission;
• Environment input promotion;
• Session creation's initial input.

Authority by operation:

Operation	Authority
SubmitInputs, ReserveEnvironmentInput	pooled sessions.Service
ExpireEnvironmentInput	pooled sessions.Service, for the Worker and the Dispatcher alike. It is idempotent under the Session lock and bounded by a 5 s timeout.
PromoteEnvironmentInput, FailEnvironmentInput, ExpireEnvironmentInputs	leased ExecutionOperations (the new InputExecution family)
ListTurnInputs, GetEnvironmentInputReservation, ListEnvironmentInputWork	sessions.Reader, each in one snapshot

Deleted:

• the test-only SubmitMessage, RequestCancel and CancelEnvironmentInput (the fixture seeder now runs the real use case);
• store/scheduling.go, store.ErrExecutionAuthority and checkExecutionAuthority;
• the store versions of every moved operation.

Replay receipts, sequence and position assignment, event order, activity tracking, reservation states and expiry, and the error identities that reach the api are unchanged. PostgreSQL tests in sessionpg cover:

• concurrent same-key admission;
• steering versus a racing Turn end;
• promote/expire races;
• lease loss;
• rollback.

Checks run:

• build, go vet ./services/core/...;
• sessions and sessionpg in full with the database;
• execution and api -run for inputs;
• store -run for the input and session_initial* tests;
• tests/fixtures build, check-names.py.

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