Add changelog

CHANGELOG.md

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+# Changelog
+
+All notable changes to rtv-mod-impact-tracker are documented here. Dates are
+YYYY-MM-DD.
+
+## 1.0.0 — 2026-04-26
+
+First public release. Eight Python scripts split into two pipelines —
+game-tracking (Steam-sourced) and dep-tracking (GitHub-sourced) — that
+together tell modders which of their mods will break when the host game
+patches or an upstream library updates.
+
+**Game tracking** — for the host game you're modding:
+
+- **`snapshot.py`** — capture a decompiled-script snapshot of the current
+  Steam build into a parallel `*_history` git repo, one commit per game
+  version. Auto-detects version + buildid from `project.godot` and the Steam
+  ACF manifest; tags as `game-v<version>-build<buildid>`.
+- **`analyze_mods.py`** — diff two snapshot tags and classify each mod's
+  impact (🔴 broken / 🟡 review / 🟢 safe). Renders an HTML report with
+  inline syntax-highlighted diffs, side-by-side override-vs-vanilla
+  comparison, and a per-mod summary table.
+- **`changelog.py`** — render a Markdown changelog of game-side script
+  changes between any two snapshot tags. Useful for release-notes posts and
+  for understanding what shipped in a given patch.
+- **`fetch_version.py`** — pull a historical Steam build via DepotDownloader
+  using a manifest registry, decompile it, and commit it as a snapshot.
+  Required only for backfilling old versions; day-to-day tracking uses
+  `snapshot.py` against the live install.
+
+**Dep tracking** — for upstream mod loaders, config-menu frameworks, and
+content registries hosted on GitHub:
+
+- **`deps_fetch.py`** — sync local mirror clones of every upstream declared
+  in `[[deps]]` of `mod_tracker.toml`. Mirrors are gitignored; regenerable
+  on demand.
+- **`deps_diff.py`** — file-level + GDScript signature diff between two
+  upstream tags. Highlights added / removed / renamed / signature-changed
+  functions so the breaking-change surface is obvious at a glance.
+- **`deps_audit.py`** — scan your own mods' call sites against a dep diff
+  and flag anything that touches a changed API. Outputs a punch list of
+  mod files + line numbers worth reviewing before bumping the upstream
+  version requirement.
+- **`deps_changelog.py`** — Markdown release notes summarising upstream
+  changes between two tags. Drop-in for the dep's own release post or for
+  internal "what's new" briefings.
+
+**Configuration & infrastructure**:
+
+- **TOML-driven** — single `mod_tracker.toml` declares game paths, Steam
+  app id, and the list of upstream `[[deps]]`. All scripts read from it,
+  so adding a new dep is a one-block config change.
+- **HTML reports** with inline syntax highlighting (Pygments-via-stdlib
+  fallback) — no external CSS hosting, runs offline.
+- **Stdlib-only Python (3.11+).** No `pip install` required for normal
+  use; DepotDownloader is the only external binary, and only `fetch_version.py`
+  needs it.
+- **MIT licensed.** Use it, fork it, ship it.