Doc step-B2 handler-merge + prompt-io

Split from the step-B2 code commit to keep the runtime diff
free of `ai/` meta noise,

- `ai/conc-anal/ria_nursery_removal_plan.md`: add a "Step-B2
  outcome" section — the dead-outer-`raise` trace, why the
  merge is behavior-preserving, and the gate results.
- `ai/prompt-io/claude/20260702T222544Z_9201a2ed_*`: NLNet
  provenance (log + unedited raw) for the step-B2 work.

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

ai/conc-anal/ria_nursery_removal_plan.md

@@ -229,6 +229,53 @@ Gate (trio backend, all 0-failure):
   completing within an 800s bound — but split across the
   above three runs EVERY module passed under step B.
 
+## Step-B2 outcome (2026-07-02, done in tree)
+
+Step B committed as `9201a2ed` (code) + `d2e812fb` (docs), then
+branched to `drop_ria_nursery`. Step B2 (the deferred
+handler-merge) implemented on top (uncommitted):
+
+- the two nested handlers in
+  `_open_and_supervise_one_cancels_all_nursery` collapse to
+  ONE `except BaseException as _scope_err` + the existing
+  `finally`. The `outer_err`/`inner_err` locals go away.
+
+Why it's safe (trace, not hope): the OLD inner handler records
+`errors[actor.aid.uid]` as its FIRST statement (before any
+await). So whenever an error path runs, `errors` is non-empty.
+The OLD outer handler was only reachable via leakage from the
+inner handler (it catches `BaseException`, so nothing from the
+`yield` scope bypasses it) — and by then `errors` is already
+populated, so the `finally`'s `raise` from `errors` ALWAYS
+superseded the outer handler's own `raise`. i.e. the outer
+`raise` was dead. The outer handler's other effects
+(`_scope_error`, a 2nd debugger-wait, child-cancel) are
+redundant with the merged handler + `finally`. So one handler
++ `finally` is observably equivalent.
+
+Residual nuance (accepted): in the rare "`trio.Cancelled`
+delivered during the non-shielded `maybe_wait_for_debugger`"
+path, the merged form may leave `_cancel_called` False (cancel
+happens after the wait), so `open_nursery`'s tb-hiding guard
+(`not cancel_called and _scope_error`) can show a tb it
+previously hid. More informative, not less; no test asserts on
+it.
+
+Gate (box ran ~2.7x slow this session, load-induced
+`TooSlowError` flakiness on timing tests — NOT code; see
+[[env_cpu_throttle_masquerades_as_regression]]):
+- baseline (pre-B2 tip `9201a2ed`) full suite
+  (`-k 'not dynamic_pub_sub'`) = 300 passed + 1
+  `test_ext_types_over_ipc` `TooSlowError` that passes 6/6 in
+  isolation (4.89s).
+- B2 error/cancel gate (`test_cancellation remote_exc_relay
+  inter_peer_cancellation advanced_faults oob_cancellation
+  to_actor spawning local rpc`) = 71 passed, 1 xfailed
+  (125s).
+- B2 full-suite run: see `b2_full.log` (result appended on
+  completion). RECOMMEND a clean full-suite run on a
+  normal-speed box before this merges.
+
 ## Verification gate
 
 - `tests/test_cancellation.py test_spawning.py test_local.py

ai/prompt-io/claude/20260702T222544Z_9201a2ed_prompt_io.md

@@ -0,0 +1,68 @@
+---
+model: claude-opus-4-8
+service: claude
+session: c7860302-6a33-4d92-821c-0807d68be464
+timestamp: 2026-07-02T22:25:44Z
+git_ref: 9201a2ed
+scope: code
+substantive: true
+raw_file: 20260702T222544Z_9201a2ed_prompt_io.raw.md
+---
+
+## Prompt
+
+Continuing on branch `drop_ria_nursery` after step B
+(`9201a2ed`). User green-lit the deferred error-handler merge:
+
+> yes, i checked us out on a new branch for the ria-tn removal;
+> work off the new wkt branch drop_ria_nursery.
+
+and (re the migration) "nope, later we'll do this", plus:
+
+> do ensure the test suite is green in the current state and
+> those going forward ya?
+
+and the standing reminder: no commits without a human ack.
+
+## Response summary
+
+Step B2 of the `._ria_nursery` removal (issue #477): merge the
+two nested error handlers in
+`_open_and_supervise_one_cancels_all_nursery` into one, now
+that the 2ndary nursery (whose `__aexit__` the outer handler
+originally guarded) is gone.
+
+Trace-driven justification (see
+`ai/conc-anal/ria_nursery_removal_plan.md` Step-B2 section):
+the old inner handler records `errors[uid]` as its FIRST
+action, so `errors` is always non-empty by the time control
+could reach the outer handler — making the outer handler's
+`raise` DEAD (always superseded by the `finally`'s
+raise-from-`errors`). Collapsing to a single
+`except BaseException as _scope_err` + the existing `finally`
+is therefore behavior-preserving.
+
+- removed the outer `try` + the
+  `except (Exception, BaseExceptionGroup, trio.Cancelled)`
+  handler; de-indented the inner handler one level.
+- dropped the now-unused `outer_err`/`inner_err` locals
+  (single `_scope_err`).
+- net ~30 lines lighter.
+
+Gated hard: baseline (pre-B2 tip) = 300 passed + 1
+load-induced `TooSlowError` flake (passes 6/6 in isolation);
+B2 error/cancel gate (`test_cancellation remote_exc_relay
+inter_peer_cancellation advanced_faults oob_cancellation
+to_actor spawning local rpc`) = 71 passed, 1 xfailed; B2
+full-suite run following.
+
+## Files changed
+
+- `tractor/runtime/_supervise.py` — collapse the two handlers
+  into one; drop `outer_err`/`inner_err`
+
+## Human edits
+
+None yet — committed via the drafted
+`.claude/git_commit_msg_ria_b2.md` (user-driven
+`git commit --edit`).

ai/prompt-io/claude/20260702T222544Z_9201a2ed_prompt_io.raw.md

@@ -0,0 +1,55 @@
+---
+model: claude-opus-4-8
+service: claude
+timestamp: 2026-07-02T22:25:44Z
+git_ref: 9201a2ed
+diff_cmd: git diff 9201a2ed..drop_ria_nursery
+---
+
+# Raw AI output (diff-ref mode)
+
+Step-B2 code lives on `drop_ria_nursery` after `9201a2ed`; per
+diff-ref mode the verbatim content is reachable via the pointer
+below.
+
+## Generated files
+
+> `git diff 9201a2ed..drop_ria_nursery -- tractor/runtime/_supervise.py`
+
+`_open_and_supervise_one_cancels_all_nursery`:
+- removed the outer `try:` wrapper and the
+  `except (Exception, BaseExceptionGroup, trio.Cancelled) as
+  _outer_err:` safety-net handler.
+- the former inner `except BaseException` is now THE handler,
+  renamed local `_inner_err` -> `_scope_err`, de-indented one
+  level; it sets `an._scope_error`, records `errors[uid]`,
+  waits on the debugger, `_join_procs.set()`, then a shielded
+  classify/log + snapshot-ria + `an.cancel()` + 0.5s-bounded
+  `_reap_ria_portals()`. No re-raise (the `finally` raises
+  from `errors`).
+- `finally` block unchanged.
+- dropped the `outer_err`/`inner_err` local decls at fn top.
+
+(The diff is large — ~119+/149- — because de-indenting the
+handler body one level rewrites every line in the block; the
+logic delta is just "two handlers -> one".)
+
+## Test runs (verbatim)
+
+```
+baseline (pre-B2, step-B tip 9201a2ed), full suite
+(dynamic_pub_sub deselected):
+1 failed, 300 passed, 9 skipped, 2 deselected, 1 xfailed,
+2 xpassed in 1499.49s
+  -> the 1 failure = test_ext_types_over_ipc[...] trio.TooSlowError
+     (load-induced; passes 6/6 in isolation in 4.89s)
+
+B2 error/cancel gate:
+tests/test_cancellation test_remote_exc_relay
+test_inter_peer_cancellation test_advanced_faults
+test_oob_cancellation test_to_actor test_spawning test_local
+test_rpc
+-> 71 passed, 1 xfailed in 125.26s
+
+B2 full-suite run: see b2_full.log
+```