Add .trionics.start_or_cancel() test suite

[goodboy]

Resolve #474 with a new tests/trionics/test_taskc.py (9 tests) covering the trio.Nursery.start() wrapper landed in PR #464, incl. the modden.runtime.progman.open_wks() use case dug out as a minimal repro.

Deats,

• the lossy RuntimeError('child exited without calling task_status.started()') only fires when the child absorbs its ambient cancel pre-.started() (graceful-teardown pattern); a well-behaved child surfaces Cancelled direct from .start() on trio 0.29 - verified empirically 1st.
• test_sibling_err_not_masked_by_startup_rte: the modden case; ONLY the root-cause sibling ValueError escapes the nursery with the wrapper vs. bare-.start()'s lossy riding-along startup-RTE noise.
• test_pure_oob_cancel_not_morphed_to_rte: plain ancestor cs.cancel() exits clean vs. bare's eg-wrapped RTE.
• test_genuine_startup_rte_still_raised: sans cancellation the protocol-bug RTE re-raises same as bare.
• test_childs_own_rte_never_demoted_to_cancel: exact-msg + isinstance-guard regression cover; a child's own RuntimeError('never got started!')/RuntimeError(1234) never demotes to a Cancelled.
• test_started_value_and_args_passthru: positional args, name= and .started()-value forwarding.

Also,

• use_start_or_cancel=False params pin upstream trio's current lossy behaviour as wart-documentation: a break on a trio upgrade likely means new upstream porcelain and the wrapper deserves a re-audit.
• verified 0 flakes over 50 hammer runs + 2 impl mutations each caught by exactly the targeted tests.

Prompt-IO: ai/prompt-io/claude/20260702T161624Z_65bf9df5_prompt_io.md (this patch was generated in some part by [claude-code][claude-code-gh]) [claude-code-gh]: https://github.com/anthropics/claude-code

[Copilot]

Pull request overview

Adds a dedicated unit-test suite for tractor.trionics._taskc.start_or_cancel() to prevent regressions around Trio’s Nursery.start() startup/cancellation edge cases (the issue #474 / PR #464 motivation), plus accompanying prompt-IO artifacts.

Changes:

• Add tests/trionics/test_taskc.py covering out-of-band cancellation, sibling-error cancellation, genuine startup-protocol errors, and argument/started() value passthrough.
• Include use_start_or_cancel=False parametrizations to explicitly pin (and document) current upstream Trio behavior alongside the wrapper behavior.
• Add prompt-IO markdown artifacts under ai/prompt-io/claude/ describing the generation/session context.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/trionics/test_taskc.py	New test module validating start_or_cancel() behavior across cancellation/error scenarios and passthrough semantics.
ai/prompt-io/claude/20260702T161624Z_65bf9df5_prompt_io.raw.md	Raw prompt-IO capture for the test generation session.
ai/prompt-io/claude/20260702T161624Z_65bf9df5_prompt_io.md	Summarized prompt-IO metadata and rationale for the added test suite.

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