core: RetryPolicy total sleep budget (maxTotalDelay) and builder - #78

Summary

• RetryPolicy bounded each sleep (maxDelay, also the Retry-After ceiling) and the attempt count, but nothing bounded their sum: raise maxDelay to bridge a per-minute window and maxAttempts to five, and one call can hold the caller's thread for minutes of sleeping. ADR-027 (plan decision D3) adds maxTotalDelay — default 1 min, the worst case the other defaults already allowed, so behaviour at the defaults is unchanged — checked before every sleep: a sleep that would exceed the budget is never started, retrying ends and the exception surfaces with its hint preserved, mirroring the ADR-025 ceiling. Exactly-reached budgets are honoured.
• Breaking (pre-1.0, ADR-019): the canonical constructor gains a fourth Duration after maxDelay (seven components). Migration: RetryPolicy.builder() (new — starts from defaults(), one setter per knob, the path that survives future knobs), or insert Duration.ofMinutes(1) as the fourth argument. No compatibility constructor, by decision: it would keep the positional trap public beside the builder meant to end it. Zero external consumers today.
• Starter: fanar.retry.max-total-delay (default 1m); fanarRetryPolicy is built through the builder, so the four knobs validate together and a max-delay raised above the budget fails the context at startup with the policy's own message. ADR-014 / ADR-020 amended; ARCHITECTURE, API_SKETCH §10–11, sample application.yml, PROJECT_STATE and CHANGELOG updated.

Type of change

• Bug fix
• New feature / capability
• New module
• Refactoring (no behavior change)
• Breaking change
• Documentation / tooling / CI only

Test plan

• mvn verify passes locally
• Live tests pass against the real Fanar API (if applicable, with FANAR_API_KEY set)
• GraalVM --self-test passes (if the SDK's reflective surface changed)

Reviewer checklist

• [ x] Public API changes are intentional, documented in javadoc, and an ADR was added/updated if the design changed
• Which *IntegrationTest proves the behaviour this PR claims? — FanarClientRetryIntegrationTest.retryAfterHintsBeyondTheTotalBudgetEndRetrying (public builder → chain → scripted server: two 429s with Retry-After: 1 under a 1 s ceiling and 1 s budget — first slept, second refused, hint preserved, fanar.retry_count=1, no further request); loop semantics in RetryInterceptorTest (totalDelayBudget*, retryAfterHintBeyondTheRemainingBudget*); starter in FanarAutoConfigurationTest (retryKnobsAreValidatedTogetherAtStartup, knob default + override)
• Breaking changes carry @Deprecated(since, forRemoval = true) and a migration path in the PR description
• BOM updated if modules were added or removed
• No third-party types leak into qa.fanar.core public API
• If a non-obvious project decision was made, capture it in memory/ so future contributors find it