0528 — pkg/logger, pkg/retry, pkg/errors, pkg/context merge into a flat pkg/common

Context

Five tier-zero packages sat beside pkg/common: logger (126 importers), retry (84), errors (7), context (1), concurrency (5). Nearly every file imported the common+logger+retry trio; pkg/errors and pkg/context shadowed stdlib names, forcing vulkanerrors/vulkanctx aliases. The domain-vs-plumbing seam cuts through retry itself (RetryPolicy is domain data serialized inside MessageOptions; the executors are plumbing), so a two-package split would keep a cross-import — and common already imported retry.

Decision

• Flat merge into pkg/common; subpackages rejected (common/logger is still package logger to Go — path-only grouping changes nothing callers see).
• Renames: retry.Policy -> common.RetryPolicy; retry.DatastoreRetry -> common.RetryDatastore (user-set, matches the Datastore scheme) with NewRetryDatastore; logger.With -> common.LoggerWith (bare With says nothing). Everything else keeps its name (Logger, NewDefaultLogger, Retry, sentinels, LifecycleContext).
• Files named for what they declare: logger.go, lifecycle.go, errors.go, retry.go, retry_policy.go, retry_datastore.go, retry_error.go; messageoptions.go renamed message_options.go and ConcurrencyPolicy split out to concurrency_policy.go.
• concurrency stays excluded: the public-surface trim hides it entirely, so its destination is internal/, not the public vocabulary.

Consequences

• One tier-zero import; the vulkanerrors/vulkanctx alias dance is gone (labs importing examples/phase_1/common alias the pkg as vulkancommon).
• The trim’s retry demotions become unexports inside common.
• CONVENTIONS.md now reads common.Logger / pkg/common where it said logger.Logger / pkg/errors.

Narrowed by [0561] (2026-08-20): the logger group moved out to pkg/common/logging; the errors/retry/context merges stand.