Beta 2: address highest-priority code review findings

[mrfloris]

Track the highest-priority findings from the post-Beta 1 code review.

Implemented locally:

• Invalidate stale marker snapshots on level, scan-center, chunk-unload, and scan-result configuration boundaries.
• Publish live scan progress and correct full-chunk progress totals.
• Keep result-affecting settings stable throughout each scan.
• Remove avoidable renderer frame and marker allocations.
• Bound retained scan markers and prioritize sources without constructing rejected markers.
• Retain at most one loaded chunk reference during an incremental scan.
• Avoid eager configuration writes and render-only rescans.
• Align the validated marker minimum with the documented 512-marker UI minimum.
• Add focused unit regressions and pass the Gradle test/build suite.
• Complete in-client Beta 2 runtime and performance testing before release.

[mrfloris]

Additional quality-improvement pass completed locally:

• Replaced hash-based scan change detection with exact scan settings.
• Published copy-on-write config through AtomicReference and kept scanner enabled state volatile.
• Kept staged-buffer endFrame cleanup in finally.
• Moved the GPU buffer to renderer instance ownership, guarded duplicate registration, closed it from Fabric CLIENT_STOPPING, and removed the renderer-close mixin.
• Kept the no-work early exit when both marker types are hidden.
• Added allocation-free render-distance, render-section, and frustum culling before vertex generation.
• Cached static HUD components.
• Removed the empty snapshot lifecycle method and clarified currentMarkers naming.
• Added concise malformed-config warnings, unique backups, and safe default recovery, including invalid UTF-8.
• Fixed the Gradle resource task deprecation found by warning-enabled clean verification.

Verification: clean test/build succeeds with warning mode enabled; 20 tests pass with zero failures. In-client Beta 2 runtime and performance validation remains open.

[mrfloris]

Final verification update: the frustum culler was refined to use a reusable matrix-based JOML intersection instead of a deprecated Minecraft helper. A clean warning-enabled build passes, with 21 tests and zero failures.

[mrfloris]

Standards pass completed locally:\n\n- Added generation-scoped marker snapshot publication so invalidated scans cannot republish stale world or scan-area results.\n- Level, scan-center, radius, and in-range chunk-unload boundaries now advance the snapshot generation.\n- Chunk unloads immediately prune only that chunk while invalidating any in-flight scan, including when the chunk had no markers.\n- Preserved the no-flicker behavior for refreshes inside an unchanged validity scope.\n- Added focused stale-publication and chunk-invalidation tests.\n- Added the Marker snapshot glossary term and ADR 0003 to preserve the architectural tradeoff.\n\nVerification: clean test/build succeeds with all warnings enabled; 22 tests pass with zero failures. In-client Beta 2 runtime validation remains open.

[mrfloris]

Spec pass completed locally:\n\n- Independent source, flowing, and waterlogged visibility now filters retained markers from the latest published config at draw time, without waiting for a replacement scan.\n- The current marker limit is also applied from that same draw-time config snapshot.\n- Overlay disablement suppresses an extraction-stage snapshot immediately.\n- Generation-scoped retention covers the Spec requirement for movement, level changes, radius changes, and chunk unloads while preserving no-flicker refreshes inside a valid scope.\n- The documented 512-marker minimum is enforced and now asserted explicitly.\n- Added a focused regression test for independent retained-marker visibility.\n\nVerification: clean test/build succeeds with all warnings enabled; 23 tests pass with zero failures. In-client Beta 2 runtime testing remains open.

[mrfloris]

Beta 2 reliability/performance checklist audit completed locally.\n\nAlready complete and reverified: generation cancellation, immediate visibility and marker-limit filtering, live progress/chunk reporting, bounded source-priority accumulation without rejected marker allocation, one active chunk reference, frustum/distance/section culling, and close-time config persistence. Existing tests cover partial snapshot replacement, stale-generation rejection, marker caps/source priority, progress math, culling, and malformed-config recovery.\n\nRemaining partial item completed in this pass:\n\n- Added an immutable RenderSettings snapshot with predecoded source, flowing, and outline colors plus precomputed alpha/border values.\n- Publish user config and its render projection together through one AtomicReference state.\n- Renderer now consumes only that immutable snapshot and performs no per-frame color decoding or config copying.\n- The public config accessor returns a defensive copy so callers cannot mutate published runtime state.\n- Added regression coverage proving render snapshots remain immutable and retain their predecoded values.\n\nVerification: clean test/build succeeds with all warnings enabled; 24 tests pass with zero failures, and diff whitespace validation is clean. In-client Beta 2 runtime/performance testing remains open.