Finish the spec iteration: stream its lines and document the set stage

[peterbjohnson]

t16 (spec iteration against coverage, with a second document of the set as the check) reached its cost cap with its reviewer's last two changes open. The work is on its branch; this ticket finishes it and nothing more:

1. iterate_spec in in2lambda_agent/pipeline.py accumulates its stage lines and run replays them after the loop returns, so the page (ui/server.py streams on_stage) shows ocr and then nothing through three model calls and six spec runs, then eleven lines at once. Give iterate_spec the callback (result.add_stage or an on_stage parameter) and report each line as it is made; a test in tests/test_pipeline.py asserts the order in which on_stage sees them.
2. tests/test_docs.py's stage guard collects add_stage calls with a string constant only, so the set stage added with a variable is invisible to it, and set appears in docs/how-it-works.md as a corpus column anyway. Make the guard see the stages iterate_spec emits and match a stage's own section, not any backticked word; then correct the page: ten stage names not nine, a set row in the table, and the model-call section says what the spec iteration's calls are given (the coverage of each try, and the second document's) and how many there may be (--tries).
   Full suite green, README's run section names --tries. Done when the page streams every line as it happens and the docs test fails if a stage is undocumented.

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Workbench ticket t28.