refactor(subscriptions): drop the vendored finite state machine DSL

[yordis]

• The fsm package the DSL came from is unmaintained (last release 2017) and its author now advises against using it, recommending plain structs with multiclause pattern matching instead.
• Vendoring a copy in chore: support only the last three Elixir and OTP releases #16 was only a stopgap so the compiler would stay quiet on Elixir 1.19 and 1.20; carrying generator code we cannot maintain is worse than carrying the clauses it generated.
• Removing the macro layer makes the subscription state transitions readable and greppable, which matters because this module drives catch-up and acking.

[coderabbitai]

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Configuration used: Organization UI

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Plan: Advanced

Run ID: 122dbde7-89b9-434d-b805-c0300f66ca38

📥 Commits

Reviewing files that changed from the base of the PR and between 9215862 and 390ddbd.

📒 Files selected for processing (2)

• lib/event_store/fsm.ex
• lib/event_store/subscriptions/subscription_fsm.ex

💤 Files with no reviewable changes (1)

• lib/event_store/fsm.ex

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Walkthrough

EventStore.Fsm was removed. SubscriptionFsm now uses an explicit struct and public functions for state transitions, subscription events, catch-up handling, subscriber lifecycle operations, and stream outcomes.

Changes

Subscription FSM Migration

Layer / File(s)	Summary
Replace the FSM abstraction lib/event_store/fsm.ex, lib/event_store/subscriptions/subscription_fsm.ex	The macro-based FSM module was removed. SubscriptionFsm now defines its own struct and constructor.
Implement subscription transitions lib/event_store/subscriptions/subscription_fsm.ex	Explicit function clauses handle subscription, catch-up, acknowledgment, checkpoint, and notification transitions. Existing capacity, disconnected, and stream-read outcomes remain supported.
Implement subscriber lifecycle lib/event_store/subscriptions/subscription_fsm.ex	Connection, disconnection, and unsubscribe functions preserve subscriber cleanup, checkpoint persistence, and final unsubscribe behavior.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~30 minutes

Change: Refactor

Merge Risk: ⚪ Minimal · up to 390dd

No concrete regression remains that should block merging.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 24 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: removing the vendored finite state machine DSL from subscriptions.
Description check	✅ Passed	The description explains why the FSM DSL was removed and how explicit subscription state transitions replace it. It directly matches the changeset.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yordis/refactor-replace-fsm-dsl

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_{A rabbit hops through states anew
With structs lined up in ordered queues
Events now follow functions clear
Catch-up paths stay close and near
Subscribers disconnect, checkpoints cheer}

_{Comment @coderabbitai help to get the list of available commands.}

[cursor]

PR Summary

Medium Risk
Touches subscription catch-up, acking, and reconnection logic in a behavior-preserving refactor; regressions would affect event delivery guarantees rather than compile-time-only changes.

Overview
Removes the vendored EventStore.Fsm macro DSL and reimplements SubscriptionFsm as a plain struct with multiclause functions that pattern-match on state and data.

State transitions that used next_state / respond now return an updated %SubscriptionFsm{} or {reply, fsm} for ack errors, matching how Subscription already unwraps ack results. catch_up_from_stream is private, takes the FSM as its first argument, and returns the struct instead of going through the macro runtime. The external entry points (subscribe, catch_up, ack, checkpoint, notify_events, etc.) stay the same so the GenServer call sites are unchanged.

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