fix(rivetkit): isolate engine envoys and propagate startup failures

[NathanFlurry]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[NathanFlurry]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• chore: lockfile lefthook #4647
• fix(rivetkit): surface native sqlite kv errors to callers #4645
• fix(rivetkit): prevent sleep races during disconnect and db work #4644
• fix(rivetkit): isolate engine envoys and propagate startup failures #4643 👈 (View in Graphite)
• test(rivetkit): remove wasm sqlite-only fixtures and coverage #4642
• chore(sqlite-vfs): remove wasm sqlite packages and workspace wiring #4641
• chore(rivetkit): remove wasm sqlite runtime #4614
• fix(sqlite-native): restore native startup kv preload #4639
• fix(sqlite-native): delete metadata before chunk range #4638
• fix(sqlite-native): keep truncate cache coherent #4637
• fix(sqlite-vfs): use delete range for truncate cleanup #4636
• fix(sqlite-native): restore kv error hook #4635
• perf(sqlite-native): avoid cloning cached read chunks #4634
• perf(sqlite-native): gate kv operation labels #4633
• perf(sqlite-native): reuse read cache for partial writes #4632
• perf(sqlite-native): remove delete file existence probe #4631
• fix(sqlite-native): gate native read cache behind env var #4630
• fix(pegboard): isolate runner config dc lookup failures #4625 : 1 other dependent PR (#4626 )
• docs(pegboard): note runner config upsert split write risk #4624
• fix(pegboard): unpack global runner config keys correctly #4623
• fix(pegboard-gateway): enforce tunnel message state #4622
• fix(pegboard-runner): clear terminal tunnel routes #4621
• fix(pegboard): skip protocol version keys in runner pool backfill #4620
• fix(pegboard): create missing normal runner configs #4619
• fix(epoxy): reuse cached optimistic value #4618
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[claude]

PR Review: fix(rivetkit): isolate engine envoys and propagate startup failures

This is a focused, well-scoped fix addressing two related issues: envoy singleton reuse across drivers and silent startup failure swallowing.

---

What this PR does

1. Envoy isolation - Prevents EngineActorDriver instances from sharing a global singleton envoy handle. Without this, multiple drivers in the same process silently reuse the first driver connection regardless of config.
2. Startup failure propagation - Changes started() from async fn started returning void (errors silently swallowed) to returning anyhow::Result<()>, threading the error through the N-API bridge and into TypeScript via .reject(error).

---

Issues

Medium: start_envoy panics on startup failure instead of returning Result

Per CLAUDE.md conventions, failable functions should return anyhow::Result and use .context() rather than .expect(). A panic here is not recoverable and bypasses the error propagation the rest of this PR correctly establishes. Suggested fix: change the return type to anyhow::Result and replace .expect() with .context("envoy failed to start")?.

Minor: Prefer .context() over anyhow::anyhow!()

CLAUDE.md states: prefer .context() over the anyhow! macro. The .map_err call in handle.rs can be replaced with .context("envoy stopped before startup completed")?.

---

Positive observations

• notGlobal: true is the highest-impact change, correctly scopes envoy handles to each driver instance.
• TypeScript rejection path in actor-driver.ts is now correct; the previous .then callback with no rejection handler silently dropped startup errors.
• N-API bridge in envoy_handle.rs correctly chains .map_err() to surface Rust errors to JS as napi::Error.
• No usage of rivet.gg, forbidden concurrency primitives, println or eprintln macros, or Mutex over HashMap.