v0.4.0: Umbrella - move Go implementation out of repo root

[tazarov]

Objective

Track the v0.4.0 repository cleanup that moves Go implementation code out of repo root while preserving public import compatibility.

Why

Current mixed-language root layout makes long-term maintenance and Java binding expansion harder. This effort makes Go implementation ownership explicit and keeps root API stable.

Tracking Checklist

• v0.4.0: Move Go implementation into subtree (non-breaking) #39 Move Go implementation into subtree (non-breaking)
• v0.4.0: Add thin root facade to preserve import compatibility #40 Add thin root facade to preserve import compatibility
• v0.4.0: Reorganize tests for subtree layout + API compatibility #41 Reorganize tests for subtree layout + API compatibility
• v0.4.0: Update Make/CI for relocated Go package layout #42 Update Make/CI for relocated Go package layout
• v0.4.0: Refresh docs/examples after Go subtree move #43 Refresh docs/examples after Go subtree move
• v0.4.0: Add compatibility gate checklist for root cleanup refactor #44 Add compatibility gate checklist for root cleanup refactor

Guardrails

• Keep module import path stable for current users.
• Keep public method-level API behavior stable unless explicitly approved.
• Ensure make test and make lint pass before merge.

Done Definition

• Implementation code no longer primarily lives at repo root.
• Root package is a thin compatibility facade.
• CI and docs fully updated.
• Compatibility checks completed and noted in release notes.

[tazarov]

All sub-issues completed (#39–#44): Go implementation relocated to internal/ subtree, root facade in place, tests reorganized, CI updated, docs refreshed, compatibility gate added.