Fix NPE and inverted numeric guidance

[mprimi]

Note for reviewers: this PR consists of 4 atomic commits, and may be easier to review them individually, in order.

1. Gitignore FFI generated files
2. Add tests, some document existing behavior, like NPE on null details. Others fail, I think correctly so.
3. Fix the numeric guidance, turns corresponding tests green
4. Fix NPE and do a little refactoring around it, all tests are green

Summary

Fixes two bugs in the public Assert numeric/comparison guidance helpers, adds test coverage for the Assert public API, and stops tracking generated FFI files.

The bugs were found by adding tests for the Assert public interface and cross-checked against the reference Go and Rust SDKs.

Bug 1 — numeric guidance maximize was inverted for sometimes* helpers

The four sometimes* numeric comparison helpers emitted the same maximize direction as their always* counterparts. Per the reference SDKs, for a given operator the sometimes variant must use the opposite direction — so the platform was being steered the wrong way for these assertions.

Method	Before	After	Go SDK	Rust SDK
alwaysGreaterThan / …OrEqualTo	false	false	false	false
alwaysLessThan / …OrEqualTo	true	true	true	true
sometimesGreaterThan / …OrEqualTo	false	true	true	true
sometimesLessThan / …OrEqualTo	true	false	false	false

References:

• Go: assert/rich_assert.go (guidanceFnMinimize → maximize=false, guidanceFnMaximize → maximize=true) and assert/numeric_guidance.go
• Rust: lib/src/assert/macros.rs and lib/src/assert/guidance.rs

Bug 2 — comparison helpers mutated the caller's details and threw on null

The numeric and boolean-map helpers merged guidance into details via details.setAll(guidanceData), which (a) mutated the caller-supplied ObjectNode in place (clobbering same-named keys, leaking across reused instances) and (b) threw NullPointerException on a null details argument — unlike the plain always/sometimes/etc. methods, which tolerate null.

Fix: a single mergeGuidance(details, guidanceData) helper that merges into a null-safe deepCopy(), leaving the caller's object untouched (at the cost of one additional copy).
Emitted-details content is unchanged (guidance keys still win on collision).

Also hoisted the per-call new ObjectMapper() allocations to one static final MAPPER.

Tests

Adds coverage for the Assert public API (previously smoke-tested only). A test-only CaptureSupport reflectively installs an in-memory output handler (output otherwise goes to a NoOpHandler in unit tests) and resets the static dedup trackers between tests — no production code was changed to enable testing.

Covers: the assert-type/display-type/condition/hit/must_hit matrix, the first-pass/first-fail dedup contract, numeric & boolean guidance (direction, strictly-better gating, NaN carve-out), rawAssert/rawGuidance passthrough, details non-mutation, null handling, and concurrent dedup.

Misc.

Adds generated FFI files (ffi/swig, generated FfiWrapperJNI.java, generated resources) to .gitignore.