Closes audit Finding 6 (2026-05-20).
Pre-fix ``_STOP_TOKENS = frozenset({"it", "to", "word"})`` was
hardcoded inside ``generate.stream.generate()`` and inhibited those
three tokens unconditionally across every pack, every language, and
every domain. If a pack legitimately needed one of them as a content
word — e.g. a philosophy pack where ``"word"`` maps to λόγος, or a
syntax pack where ``"to"`` is a content node — there was no override
path. The ``_try_index`` guard handled the case where the token was
absent from the pack, but offered nothing for packs that contained
the token and meant it.
Changes:
* ``generate.stream.generate`` accepts ``stop_tokens: frozenset[str]
| None = None``. ``None`` resolves to the historical
``_STOP_TOKENS`` constant, preserving byte-identity for every
pre-Finding-6 caller.
* ``RuntimeConfig.stop_tokens: tuple[str, ...] | None = None`` —
operator-level override threaded through ``ChatRuntime`` into
``generate()``.
* Default ``None`` preserves byte-identical behavior for every
existing pack and every existing test.
Scope notes:
* This PR delivers the *runtime override* surface. Manifest-driven
per-pack overrides (``generation_stop_tokens`` field in the pack
manifest) are the natural next step but require a pack-schema
ADR and re-ratification of every affected pack, so the wiring
lands first and the manifest field follows on a separate ADR.
* ``agenerate`` was identified as unreachable and is being deleted
in a sibling PR (Finding 7); its hardcoded ``_STOP_TOKENS``
reference disappears with it, so it is intentionally not touched
here.
Verification:
* 4 new tests in ``tests/test_stop_tokens_override.py``:
- ``RuntimeConfig.stop_tokens`` defaults to ``None``
- ``generate()`` signature exposes ``stop_tokens`` with default
``None``
- the historical constant is unchanged
- an explicit override flows through the runtime end-to-end
* ``core eval cognition`` — public 100/100/91.7/100, byte-identical
to the MEMORY baseline.
* ``core test --suite cognition`` — 120/0/1.
* ``core test --suite smoke`` — 67/0.
* ``core test --suite runtime`` — 19/0.
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