test: deep testing pass with coverage

[othmaneataallah]

Milestone 1: deep testing. 95 → 122 tests; 5 of 6 modules at 100% line+branch coverage.

Tooling

• JaCoCo 0.8.15 wired report-only in the parent build (per-module target/site/jacoco, never failing) + a Coverage note in AGENTS.md.

Gaps found and fixed

• core: self-equality, per-member inequality, full/absent-only toString, relative type URIs.
• jackson: invalid-textual instance fallback, non-int-convertible status ignored.
• jaxrs: new direct writer unit tests (writeability matrix, JSON/XML bytes, wildcard default, XML-failure → 500).
• registry: per-member inequality, minimal toString, plus a deterministic concurrency smoke test (parallel register/lookup, raced duplicates leave exactly one winner).
• spring: title-less conversion (locks in Spring's reason-phrase default).
• xml: empty/truncated/comment/CDATA inputs, markup-in-scalars, Boolean/Character values, nested-null omission, scripted-reader tests for defensive end-of-input guards.
• One simplification: dropped a provably-dead condition in ProblemXml.

Fixes the suite forced

• Spring forStatus pre-fills title with the reason phrase (test expectation corrected).
• Two edit slips on my side (orphaned method bodies) caught by compilation before anything else.

Known artifact (not a gap)

JaCoCo reports the Jackson non-object guard line as missed although a passing test executes it (stack-trace proven): single-statement lines that always throw execute but never complete. Documented in AGENTS.md.