fix(desktop): treat teardown as cancellation

[Phoenix500526]

Summary

Pending Session and transcript observations rejected their readiness promises
when the renderer intentionally released them during startup. Those expected
teardowns escaped the Electron IPC handlers as errors.

Distinguish caller-initiated cancellation from genuine initialization failure,
then carry an explicit ready or cancelled result across the Main/Preload IPC
boundary. Preload now stops cancelled listeners without reporting a Session as
seeded or reading a missing transcript result. Genuine seed and transcript-open
failures continue to reject unchanged.

Fixes #4434

Verification

• npm --workspace @maka/desktop run typecheck
• npm --workspace @maka/desktop run build:test
• Related Session observation and IPC suites: 68 passed
• Session observation release suite: 1 passed
• npm --workspace @maka/desktop run check:architecture: 71 fixture tests
  passed and the renderer architecture check passed
• Biome and diff checks passed for all changed files
• Manual Edit & resend/reset reproduction no longer prints the pending
  observation handler errors

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex implemented the cancellation contract and
regression tests, then reviewed and verified the resulting diff.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No