feat: overlay runtime cache configuration on defaults

[lan17]

Summary

Runtime cache configuration now overlays each cached definition's optional defaultConfig field by field. This makes a provider useful as a sparse operational control instead of forcing it to repeat the complete policy on every response.

Missing policy is now an intentional disabled baseline: request-local caching defaults off, and process-local or Redis caching remains off until that layer has an effective TTL. A configured TTL with no ramp defaults to 100%.

Closes #23.

Resolution model

Runtime value	Effective behavior
Provider returns null or defensive undefined	Use the captured baseline unchanged
Runtime field or layer leaf is omitted/undefined	Inherit the corresponding baseline leaf
requestLocal: false	Explicitly disable request-local caching
ramp[layer]: 0	Explicitly disable that shared layer
No effective shared-layer TTL	Disable that layer with policy_disabled
Effective TTL with no ramp	Use ramp 100
Provider throws or returns a malformed config shape	Fail open uncached with config_error
Runtime TTL/ramp leaf is invalid	Disable only that layer with the existing bounded validation reason

The provider is still resolved once per enabled invocation, and the resulting effective configuration is used across request-local, process-local, and Redis lookup work.

Registration and mutation safety

cached() validates static defaults when the definition is registered. Static TTLs must be positive safe integers, ramps must be finite percentages from 0 through 100, layer maps must be objects, and requestLocal must be boolean when present.

Registration captures and freezes an internal snapshot, so later mutation of the caller's defaultConfig object or maps cannot change a live use case. Dynamic overlays remain intentionally fail-open because they may come from runtime configuration systems.

Public API and migration

No constructor option or policy object was added, and defaultConfig remains optional. Existing TTL/ramp map shapes are unchanged.

Observable changes:

• Omitted DialCacheKeyConfig.requestLocal remains undefined until config resolution; its effective default is still false.
• A non-null sparse provider result now inherits omitted baseline leaves.
• Existing provider results that used {} or omission as a kill switch must use explicit requestLocal: false and/or per-layer ramp 0; DialCacheKeyConfig.disabled() returns that explicit kill-switch overlay in one call.
• DisabledReason replaces missing_config with policy_disabled; dashboards and alerts must migrate that label value.
• MissingKeyConfigError is removed from the public API. It was exported but never thrown, and missing policy no longer throws, so it is deleted rather than deprecated.
• Invalid static defaults now throw during cached() registration.

There is no cache-key, Redis-key, Lua frame, serializer, adapter ownership, or client lifecycle change.

Runtime cost

The provider call count remains one per enabled invocation. The null/undefined provider path reuses the immutable registration snapshot without a merge allocation. A non-null overlay creates one small normalized config and two bounded two-layer maps.

Validation

• 249 unit tests
• 46 Redis/Valkey integration tests
• TypeScript typecheck
• ESM/CJS build and declaration generation
• Packed ESM and CommonJS runtime behavior
• Packed consumer declaration tests, including the bounded disabled-reason union
• Node 20 packed-package compatibility smoke
• Two fresh independent review passes with no findings