feat(core): resolve and flat-merge PORC context via backend.GetContext

[muralisrini]

Adds the context (C) enrichment stage to Authorize, mirroring resource (R) resolution. Backends resolve the request context for the ordered delegation-chain subjects and the engine flat-merges the result into input[context].

• backend.Service: new GetContext(ctx, subs []string) (map, *PolicyError). Local + mock backends return {} (no external resolver); the remote backend (go-lib) will call the customer context resolver.
• Authorize: flattenSubs builds the ordered subs list from the principal -- current actor first (principal.act.sub), down the nested act chain, subject (principal.sub) last -- then resolveContext calls GetContext and flat-merges (resolved wins) into input[context]. A failed/empty resolve attaches nothing.
• new PORC keys: contextKey ("context", named to avoid shadowing the context package) and Act ("act").

Standalone/local path works now (backend returns {}); the platform path lights up once the remote backend implements GetContext (go-lib, issue C).

Tests: flattenSubs (subject-only, single/nested act chain ordering, empty, blank skipping) and flatMergeContext (merge, collision precedence, nil-tolerance, no-mutation). go build ./... + changed-package tests green.

NOTE: the principal.act shape (nested {sub, act}, current-actor outermost) has no prior use in this repo -- the engine only read sub before. Encoded explicitly in flattenSubs + its tests; confirm it matches how go-lib/sidecar builds the principal.

[codecov]

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