Inject the environment: scope bases, warn channel, fetch adapters #49

Part of #44 — spec item 6.

Scope-base lookup, the warn channel, and fetch become explicit adapters at each seam; the process environment is read in exactly one adapter at the edge.

Lands after the reshaped interfaces settle so each adapter is built once. Tests replace save/mutate/restore env dances with constructed fakes; warnings are asserted from the report interface, not console interception.