feat(libsy): let stage algorithms fall_open tier be set at runtime

[ryan-lempka]

Background: Today the stage algorithm has a fallback tier that comes from a config setting called picker. When stage algorithm is uncertain of the right model to route to it falls back to the picker. This is currently set inside of the config and fixed for the life of the process.

What: This PR adds in the ability to change the fallback tier at runtime. Introduces two new functions set_fall_open and clear_fall_open so the fallback tier is now read from session state, falling back to picker when nothing is set. Nothing calls these functions yet, so stage behavior is unchanged.

Why: A first step towards hierarchical algorithms. This will allow another algorithm (such as LLM Classifier) to set which tier the stage algorithm falls back to at some point during the session. Given a large number of stage algorithm turns will default to the picker we can use LLM Classifier to adjust it based on difficulty. Follow-on PRs will look enable doing this at the start of the session, and also at the start of a user-turn.

Summary by CodeRabbit

• New Features
  • Added persistent fall-open routing overrides for selecting the strong or weak tier.
  • Added support for clearing an override and returning to the configured default.
  • Overrides remain active without suppressing normal signal routing.

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[ryan-lempka]

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Reviewing files that changed from the base of the PR and between 053a61e and 9a9fe04.

📒 Files selected for processing (3)

• crates/libsy/src/algorithms/stage.rs
• crates/libsy/src/algorithms/util/stage.rs
• crates/libsy/src/lib.rs

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Walkthrough

The router now supports persistent fall-open tier overrides. It uses the override at zero confidence, preserves decisive signal routing, and restores the picker default after the override is cleared.

Changes

Fall-open tier routing

Layer / File(s)	Summary
Tier override state and API crates/libsy/src/algorithms/util/stage.rs, crates/libsy/src/lib.rs	Tier supports stable "strong" and "weak" labels. Public helpers set and clear the persisted fall-open tier in State.extra.
Fall-open classifier integration crates/libsy/src/algorithms/stage.rs	FallOpen selects the persisted override or picker default at zero confidence. The router uses it as its terminal classifier.
Override behavior coverage crates/libsy/src/algorithms/stage.rs	Integration coverage verifies persistence across turns, decisive signal routing, and default restoration after clearing.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 9a9fe

The change is localized to runtime fallback-tier state handling, and no actionable merge-blocking risk remains after normal checks and review.

Poem

I’m a rabbit with tiers in my tray,
Strong or weak labels guide the way.
Overrides persist through each turn,
Clear them, and defaults return.
Decisive signals still hop through—
A tidy route for me and you!

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 61.54% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 13 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: runtime configuration of the stage algorithm's fall-open tier.

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