refactor(bot): split fsm.rs into fsm/{state,status,machine,flow,encoding}

[devin-ai-integration]

Summary

bot/fsm.rs had grown to 559 lines holding five distinct concerns
behind a single file:

• lifecycle enums (DialogStatus, DialogTimeoutPolicy) with their
  wire encode/decode helpers;
• the persistent DialogState record with its netstring-based
  encode() / decode() plus builder methods;
• the self-contained DialogFlow declarative step sequence;
• the DialogMachine mutating façade over &mut dyn StateStore
  (lifecycle, timeout enforcement, metadata, flow navigation); and
• shared low-level plumbing (netstring codec, wall-clock helpers,
  session-id generator, internal metadata keys, and the encoding
  version constant).

Split along those natural boundaries per the AGENTS.md guideline
("split files when a module grows beyond ~400-600 lines or mixes
multiple responsibilities"):

• fsm/mod.rs - module wiring + public re-exports.
• fsm/status.rs - DialogStatus, DialogTimeoutPolicy, encode/decode.
• fsm/flow.rs - DialogFlow (self-contained).
• fsm/encoding.rs - netstring codec, now_unix_ms /
  duration_to_millis, generate_session_id, DIALOG_ENCODING_VERSION,
  INTERNAL_SESSION_KEY, INTERNAL_TIMEOUT_POLICY_KEY. Everything here
  is pub(super) so the encoding helpers stay invisible outside the
  fsm/ module tree.
• fsm/state.rs - DialogState + encode/decode + metadata helpers.
• fsm/machine.rs - DialogMachine lifecycle/timeout/flow operations.

External call sites are unchanged

bot/mod.rs kept its re-export:

pub use fsm::{DialogFlow, DialogMachine, DialogState, DialogStatus, DialogTimeoutPolicy};

So bot::{DialogFlow, DialogMachine, DialogState, DialogStatus, DialogTimeoutPolicy} continue to resolve bit-for-bit. The external
users (lib.rs, bot/context.rs with 26 references) compile unchanged.

Structural only, no semantic change

• Public items keep their paths (bot::*) and their public
  visibilities.
• DialogState fields remain pub (the struct was pub +
  #[non_exhaustive] before).
• Internal helpers (netstring, parse_netstring, now_unix_ms,
  duration_to_millis, generate_session_id) downgraded from
  private-to-single-file to pub(super) so siblings inside fsm/
  can call them; they were already invisible outside the file, and
  remain invisible outside the module tree.
• DIALOG_ENCODING_VERSION, INTERNAL_SESSION_KEY, and
  INTERNAL_TIMEOUT_POLICY_KEY lived as private module-level
  constants in fsm.rs; now pub(super) in fsm/encoding.rs so
  state.rs and machine.rs can reference them. Same effective
  privacy.
• Method bodies in all five impls (DialogState, DialogStatus,
  DialogTimeoutPolicy, DialogFlow, DialogMachine) are
  byte-for-byte identical to the pre-split file.
• Doc comments added on public items where missing_docs = "deny"
  started flagging them in their new homes. No behavioral effect.

Net diff: fsm.rs deleted (559 lines), six new files created
(mod.rs ~35, status.rs ~55, flow.rs ~100, encoding.rs ~75,
state.rs ~250, machine.rs ~215). Git detects
fsm.rs -> fsm/state.rs as the primary rename.

Local verification:

• cargo fmt --all --check clean
• cargo clippy --workspace --all-targets --all-features -- -D warnings clean
• cargo test --workspace --all-features -> 287 passed / 0 failed

Review & Testing Checklist for Human

• Spot-check fsm/mod.rs re-exports: bot::DialogFlow,
  bot::DialogMachine, bot::DialogState, bot::DialogStatus,
  bot::DialogTimeoutPolicy must all still resolve from
  downstream crates. bot/context.rs (26 references) is the
  biggest internal user and compiles unchanged.
• Confirm the visibility changes are acceptable: the private
  constants and helpers that were previously private to a single
  file are now pub(super) inside the fsm/ module tree. The
  struct DialogState keeps its pub fields as before; the
  DialogStatus/DialogTimeoutPolicy encode/decode helpers
  are pub(super) (previously private; they are only called by
  state.rs).
• Legacy decoder path still works: DialogState::decode retains
  the dialog|step fallback for values that predate
  DIALOG_ENCODING_VERSION = "v2". This matters for stores that
  were populated by an older bot build.

Notes

This is the next PR in the P0 structural refactor queue after #44
(dedupe can_issue_logged_in_command), #45 (extract
poll_command_completion), #46 (split client/bus.rs), and #47
(split bot/storage.rs, currently open).

Next up: client/hooks/builders.rs (574 lines),
types/entities/media_common.rs (458 lines), and the
client/backend.rs (1334 lines) + client/backend_mock.rs (1101
lines) pair. Then the P1 API tweaks (jitter, StreamTypes,
SdkErrorCode, TimeoutKind, SecretString, indexed dispatch) all
with their own tests. Finally a big #58 test-fill PR driven by
scripts/audit_teamtalk_coverage.py.

Branches from clean main; all the refactor PRs are independent
against main so they can be merged in any order. If any touch the
same file I'll rebase the later one after its predecessor lands.

The pre-existing semver CI gate is expected to remain red;
release-plz handles the eventual version bump. Every other check is
expected to pass.

Link to Devin session: https://app.devin.ai/sessions/71fdd6196cb74723a2e277bb81993a9c
Requested by: @BlindMaster24

---

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