fix(compute): terminal execution budgets and bounded generation observations

[erikleblansch]

Integrated milestone: aaeeae527f699da72f7c7be15da790a404bc54b4

The runtime tested below is 6a517b57; current head adds only the original failed-trial record. The source foundation was normally merged through #209 into main 80e7932718e79fb4c853b943bd6d5f11e50bda81; this PR now integrates the bounded diagnostic/deadline-handling milestone. Successful 4B generation remains unproven.

The original integrated Quality run 37225595367 on a5246942 failed two strict Clippy function-length limits. The follow-up extracts the existing snapshot-shape parser and Python test script without changing validation, assertions, deadlines or acceptance gates. Eight focused diagnostic tests and exact-file rustfmt/diff checks pass. Original job-log SHA-256: 1ef3aca56b4052b9b56e2f9a469543e07cfdb1718d5a8706750c286f40f6d41c. Hosted source checks now pass: Quality 37227074955, all four CodeQL analyses and the CodeQL summary. The workflow-only KVM check is skipped, not datapath evidence. The original actual OpenCode 4B trial 37228603308, attempt 1 has now failed, with exact Code 714904a3ec452b8e10d8b40be6ac85107606ca3b and core 6a517b576baa17e7329661ee0476d1848081d114. One request ends after 603225 ms with one typed execution_budget_exceeded and confirmed cleanup; no repeated request follows. Its full 4572-token prompt reaches first-forward begin at 19672 ms, but no completed first forward, generated token, tool call, code edit or test execution is observed. This proves terminal no-retry handling, not successful model execution or a solved generation stall. Original ZIP SHA-256 fabcc8ba08f47f28f85614b645679654b70f83f02c85c4f8be8722c0d1d5978c; original job-log SHA-256 6de3c4d41e1346582dc045248b25bbbf353125691f7c4e2ec191be8a8bb0e27f. The current documentation-only head 3917b2e479e96d3c02ba5b8e091ffcc75abff3d1 retains that result; all six exact-head source checks passed before the normal merge into main. The fully integrated source branch was removed; its commit history and local detached worktree remain preserved.

Integration base

Ordinary merge a5246942707890978867930dca518a29d1d48bd5 retained the main/Cloud/Signal/docs history and PR #209 quality correction f064e990. That foundation is now integrated; there was no conflict or generation-argument change. This PR targets main for source CI.

Scope

This is a private native-conversation candidate, not a successful replacement model trial.

• Negotiate execution_error_version: 1 independently of generation policy; preserve legacy clients and snapshot negotiation at admission.
• Expose execution_budget_exceeded only from typed supervisor/validated worker deadline failures after successful cleanup. Cancellation and cleanup uncertainty retain precedence.
• Add opt-in, bounded scalar observations for first-forward/token progress; no prompts, output, token IDs or raw hardware data. No model, generation-argument, owner-checkpoint or deadline changes.
• Retain original failed 4B run 37218791500 and its artifact hashes; do not claim a solved generation stall.

Checks

36 focused Rust private-conversation/service/staging tests; 8 Rust diagnostic/parser tests including real subscriber-to-fixture parsing; focused Python worker/progress and fixture tests; scoped rustfmt and diff checks pass. No model loaded or host networking changed. The separate Code consumer must explicitly negotiate the new capability before mapping this terminal failure to non-retryable HTTP 422.