feat(fs-0007): implement v0 two-client multiplayer slice

[danieldilly]

Summary

Implements FS-0007: v0 Two-Client Multiplayer Slice — the minimal end-to-end multiplayer foundation for Flowstate.

• Simulation Core (crates/sim): Deterministic World with fixed timestep, explicit tick parameter, FNV-1a 64-bit state digest, WASD movement (5.0 units/sec)
• Wire Protocol (crates/wire): Shared protobuf types (ClientHello, ServerWelcome, InputCmdProto, SnapshotProto, ReplayArtifact)
• Replay System (crates/replay): Full verification pipeline with initialization and final digest anchors
• Server Edge (crates/server): Input validation, InputSeq selection, TargetTickFloor management, LastKnownIntent fallback, session management

Trace Block

Issue: #7
Spec: docs/specs/FS-0007-v0-multiplayer-slice.md
Constitution: INV-0001, INV-0002, INV-0003, INV-0004, INV-0005, INV-0006, INV-0007, DM-0001, DM-0006, DM-0007, DM-0014, DM-0016, DM-0017, DM-0018, DM-0019, DM-0020, DM-0023, DM-0024, DM-0025, DM-0026, DM-0027, AC-0001, KC-0001, KC-0002
ADRs: ADR-0001, ADR-0002, ADR-0003, ADR-0004, ADR-0005, ADR-0006, ADR-0007

Files Changed

• crates/sim/Cargo.toml — Updated metadata
• crates/sim/src/lib.rs — Complete Simulation Core implementation (World, StepInput, Snapshot, Baseline, StateDigest)
• crates/wire/Cargo.toml — New crate for shared wire protocol types
• crates/wire/src/lib.rs — Protobuf message types (prost derive)
• crates/replay/Cargo.toml — New crate for replay recording/verification
• crates/replay/src/lib.rs — AppliedInput, ReplayRecorder, verify_replay()
• crates/server/Cargo.toml — New crate for Server Edge
• crates/server/src/lib.rs — Server main module (session lifecycle, stepping, finalization)
• crates/server/src/session.rs — Session management
• crates/server/src/validation.rs — Input validation (NaN/Inf, magnitude, tick window, rate limit)
• crates/server/src/input_buffer.rs — Input buffering with InputSeq selection

Determinism / Simulation Impact

• This PR changes simulation-plane logic
  • Impact: Establishes the deterministic foundation. World::advance() is pure (no I/O), uses explicit tick parameter per ADR-0003, and produces identical StateDigest for identical inputs.
  • Verification: test_t0_04_determinism_multiple_runs verifies digest equality across fresh World instances. test_t0_09_replay_verification_passes validates full replay path.

Verification

• just ci — All 58 tests pass (sim: 16, wire: 6, replay: 8, server: 28)
• just ids — Constitution IDs valid
• just spec-lint — Spec lint passed

Test Coverage (Tier-0 Gates)

Gate	Test
T0.1	test_t0_01_two_client_handshake
T0.2	test_t0_02_join_baseline
T0.4	test_t0_04_wasd_deterministic_movement, test_t0_04_determinism_multiple_runs
T0.5	test_t0_05_advance_takes_explicit_tick, crate isolation
T0.5a	test_t0_05a_tick_floor_relationship
T0.7	test_t0_07_malformed_inputs_no_crash
T0.8	test_t0_08_replay_artifact_has_required_fields
T0.9	test_t0_09_replay_verification_passes
T0.10	test_t0_10_initialization_anchor_failure
T0.11	test_t0_11_future_input_buffered
T0.12	test_t0_12_lki_determinism, test_t0_12_lki_fallback, test_t0_12_empty_inputs_deterministic
T0.12a	test_t0_12a_noncanonical_input_order
T0.13	test_t0_13_inputseq_tie_uses_lki, validation tests
T0.13a	test_t0_13a_floor_enforcement_recovery
T0.14	test_t0_14_disconnect_handling
T0.15	test_t0_15_match_termination
T0.16	test_t0_16_connection_timeout
T0.17	test_t0_17_playerid_test_mode, test_t0_17_playerid_non_assumption
T0.18	test_t0_18_floor_coherency_broadcast
T0.19	test_t0_19_wire_crate_exists (shared crate structure)
T0.20	just ci passes

Risk / Notes

• Networking not integrated: This PR provides server-side logic in "manual step mode." Actual ENet transport layer is a follow-up (Tier-1).
• No Game Client: Client implementation is deferred. Tests simulate client behavior via server API.
• Cross-platform replay: Deferred per ADR-0005 (v0 scope is same-build, same-platform).

Checklist

• Trace block is complete and accurate
• This PR stays within Constitution invariants (no silent weakening)
• No new dependencies added, or licensing intake updated under docs/licensing/
• Spec exists for non-trivial changes
• Gates added per spec's Gate Plan